Shooters are usually heavy with online, and the 360 has the upper hand when it comes to actually playing games online. I'm not saying it's better or worse, there are just more people. How many time have you heard the line: "I'm getting it on the 360 because all of my friends are"? 
 
For me personally, it's the controller though. I HATE the loose feel and strange placement of the analog sticks on the PS3 controllers.

I am not bashing the PS3 I own one and other genres seem to do fine with Stealth dominated by MGS4 and action games doing well with Uncharted and several other games selling well.  However the PS3 is only a few million globally behind the 360 in consoles sold and has some solid shooter's with Killzone, the Resistance series, and now MAG but it seems like they can never get a game that can compete with Halo or Gears sales wise even though the PS3 games are as good or close to the 360 games.

how are they struggling?  360 has Halo...what else (Left 4 Dead just came to mind, there's 2)?  If you mean mutliplats then more people buy those for 360 because more people have a 360 or prefer their multiplats on 360.
 
I personally enjoy the Resistance series more than Halo or even the last couple of Call of Duty games.  Then there is Killzone 2 which was a top notch FPS but, for whatever reason, I still feel like people seem to underrate it.  I guess the hype for it for such a long time made a lot of people want to hate on it no matter what.  Much like what happens with anything that gets too much attention.  
 
But regardless, I dunno what you mean by struggling still.  Didn't Resistance and Killzone sell well?  And both series you will have no problem at all finding people to play with online (not even an issue that should cross your mind).

I voted for "controller" because that affects both offline and online shooters. But I certainly would not buy a PS3 game emphasizing team work, unless a headset were to be bundled. That's my main problem with the whole thing. They could have totally thrown it in with the Slim too.

I don't think I'd say the PS3 is "struggling," but personally I far prefer the 360 for shooters solely because I far prefer the 360's big, chunky to the PS3's.

I recently bought a PS3, along with MAG and I don't feel like it is struggling at all.  I have had TONS of fun online with no end in sight, which is a fucking great thing lol.     
 
Could just be because its new, but man I love it.

Controller? What?  
 
The 360's controller is ass. The PS3's... the best. 
 
I don't know, maybe because I've been using the Dualshock model for a long ass time.

thumbstick placement sucks ass imo.

I don't think shooters struggle on ps3, i just think they sell more on xbox because it seems to have more of a shooter fan base on it.

I am not saying the PS3 is not good for shooters I really enjoy Resistance and thought Killzone 2 was good it just made me literally nauseous so I couldn't play it.  However even in multiplat sales wise they don't do as well.  For example I got this from Wikipedia   According to the NPD Group, Modern Warfare 2 sold approximately 4.2 million units for the Xbox 360 and 1.87 million units for the PlayStation 3 in the U.S. during the month of November 2009  Whereas the 360 has sold 39 million consoles worldwide to the PS3 31 consoles and I'm not even going to deny that some of the 360s numbers are double or triple purchases from the same person I am on number 2 purchased. The number of consoles is close but the numbers of copies of the MW2 game is really not.

360 controller feels like it's made for shooter. PS3's trigger design is detrimental to FPS games. 

There's a reason I paid 60 damn bucks to connect a 360 wired controller to my ps3 (first and third person shooters are unplayable on the dual shock unless you have child size hands imo), still then games that don't let you program shooting to the back triggers means using rb/lb....yeah right back to the store you go.

"I'm getting it on Xbox because all my friends have it." 
I think the controller is a big factor too, but personally I like the PS3's. The Xbox's sticks feel more built for an FPS though.
However, I prefer the shooting buttons on the PS3 a lot more (I know, call me crazy, but I honestly don't care about how "immersive" the buttons feel. A button is a button, and the Xbox's triggers and bumpers feel more resistant than the PS3's). 
Also, I have pretty small hands *insert crude joke here* and the PS3 controller falls more naturally into mine. The Xbox feels nearly the same though.

little bit of each but mostly the controller.  it's just not made for shooters. 
if i want to play a shooter i'll most likely get it on PC, because i like the kayboard and mouse controls better. 
 
the PS3 controller is better for games like god of war, uncharted, ratchet and clank, etc

a lot of the kinda of people who enjoy shooters online prefer them on pc, or on 360, so i think it really does just boil down to controls. 
 
personally i prefer the ps3's sticks to the 360, and no shooter ever actually uses l2/r2 to shoot. i think most people see convex triggers and go UGH NO WAY.
 
as a ps3 owner, i don't think we'll ever see a ps3 exclusive match a 360 exclusive's sales numbers, both because of the larger number of people who own 360s, and the likelihood of them buying a 360 FOR that exclusive in the first place.
 
sony is just barely onto something with the uncharted franchise though. if they don't ruin it somehow, that could someday be the ps3's halo/gears, the reason people keep coming back. other than that i don't see anything.

The controller's terrible.

Controller.  
 
 I'm of the opinion that the 360 controller is much more ergonomic and comfortable to play with in general especially over long periods of time, I personally get much less instances of RSI.  Sony obviously hasn't changed the PS controller much since the original PS days externally but I believe its due for its evolution to a more ergonomic design. 

I think that the ps3 appeals to an audience different than the 360. More so to a slightly younger audience, and a slightly older audience, both of which could be the reason that shooters struggle.

The controller is definitely to blame.  It's just not properly designed for shooters, both analog sticks are equidistant and the triggers are too small.  It's not fun to play FPS games using a controller which seems more cosmetically pleasing rather than being ergonomically sound.

controller, r/l 2 kinda hard to keep from slipping and the sticks are too close together and the rounded top makes them slippery.

Um, I chose other because I don't find them to 'struggle' on the ps3, I think they play just fine, but it's all a matter of opinion.  i like the design of the 360's don't get me wrong, but I don't find anything wrong with the ps3's even with my big hands.

I don't get why people claim the DualShock is not suited for shooters. That controller fucking rocks in every genre.

" I don't get why people claim the DualShock is not suited for shooters. That controller fucking rocks in every genre. "

The Dualshock is beast so I don't see that as being a reason. But I don't know, I like playing FPS' on my PS3 just fine. But thinking about I think I have more on the 360 but thats just coincidence.

" I don't get why people claim the DualShock is not suited for shooters. That controller fucking rocks in every genre. "

The controller. The 360 controller just feels better for shooters.

controller. But i'm biased as i prefer the PC as my platform and the mouse and keyboard combo will never be superceeded imo.

Easy answer:  no Xbox Live.

The controller is just awful, it's an endurance challenge to hold those analog sticks.

All of the above?
 
The controller is the biggest problem though, far inferior to the 360's in my opinion. The thumbsticks are in bad places, they are way too loose, they do not have the depressions to place your thumbs like the 360 controller has. The triggers have to resistance and feel wrong, plus the fact that most games use R1 to shoot is a hell of a problem. The 360's triggers make you feel like you are squeezing a REAL trigger, not pushing a button. The DS controllers are just overall small and are probably designed for a younger audience. My hands are just uncomfortable when holding them.

I don't know what you mean by struggling. There are many good shooters for the PS3. The system just isn't drowning in them like the 360 is. For multiplats I do think more people buy those for the 360 just because the PS3 versions have a reputation for being not quite as good, which isn't always true. I do think controls and the controller are an issue as well. For whatever reason the controls generally feel off, or awkward on the PS3 due to how developers map the buttons. Bad Company immediately comes to mind here. The other issue is the fact that the basic design for the dual shock controller is over 10 years old now. It is ancient by electronic standards and what worked back then doesn't really work now. I'm talking about the placement of the analogue sticks here. Being equidistant just doesn't work that well for shooters.

They struggle? And it's definitely not the controller for me, I thought I preferred the 360 controller until I used it the other day for MW2 and found the controller very uncomfortable.

I've always considered the Playstation gamer to enjoy "variety". No one "genre" has ever really stood out on the Playstation. It's always been about variety.

Where is option  
E:No problems with PS3 shooters at all???
 
I personally love Killzone 2. Some might say the controls are a little heavy, but that was what Guerilla was going for. Besides, you can turn the "weight" off in the controller options. And since you are mentioning Gears, what about Uncharted 2? Thats an excellent shooter.

Sony needs to design a new controller made from the ground up. The PS3 controller is basically a PS1 controller with shit strapped to it. It's not terrible or anything, but the 360 controller is vastly superior in almost every way (except D-pad).

The controller isn't as tactile for shooters as the 360 console's controller.  For one thing there is the 360's triggers vs the PS3's shoulder buttons.  I know, some games allow you to flip that control to the L2 & R2 buttons, but this isn't true of all PS3 games.  In some cases, like Uncharted 2, you are stuck with using the L1 to aim and R1 to shoot (although you can swap them so L1 is shoot and R1 is aim.)  Secondly, there is the placement of the joysticks on the PS3 controller.  Having both analog sticks at the bottom is okay for a lot of other game types, but for extended hours of shooters play, the offset analogs sticks on the 360 feels more comfortable.  Thirdly, there is the wide grips beneath the 360's controller that helps a player grasp the device.  The PS3's controller is looser and not form-fitting to the natural ergonomics of a player's hand.  This may sound like I am bashing the PS3 DualShock but I am not.  I've played through both Resistance games, Killzone 2, and picked up Modern Warfare 2 for the Sony console even though I have a working 360.  I'm just pointing out that for shooters, the controller is not as well suited to the genre as the one offered by Microsoft.   

ps3 should have some cheap 20$ usb headset included with the game console when it's bought so that there isn't any excuse to not having a mic.  this seems to be the biggest problem on the PS3.

The PS3 controller is in serious need of a re-design. They have had it since the first playstation, it's not really suited for this generation. The 360 controller though is ergonomic and in my opinion it's perfect.

I love my PS3 but the controller is just Not made for FPS, I mean I can use it fine to play MW2 but The 360 controller just feels 1000x better in my hand.
 
And why can't I use the Goddamn triggers to shoot? I hate pressing a Button to shoot it feels so wrong.

Why the fuck is there no "there's nothing wrong with ps3 shooters." option?

I would guess a combination of A and then some.
 
PS3 mainly attracted RPG fans who were hoping it would feature a similar library as the PS2 - now 3 years down the road we are finally looking to be getting some of that. 
 
For me (the typical weaboo) 1 or 2 shooters a year is a plenty and even then i buy them exclusively for the multiplayer component.

They don't. Modern Warfare 2 had similar attach rate with the 360.  As far as I know, Killzone has sold well, too.
 
Out of those choices, controller :)