Setting wise:  Left 4 Dead(1): because the city settings were pretty awesome.  

Character wise:  Left 4 dead 2: In L4D1 Francis & Zoey were great characters, Bill was awesome and I never played without being him but I always felt Louis was a Token black guy. L4D2 Nick and Rochelle weren't that great so I avoided them, Coach was more likeable then most of the characters but what sealed the deal for me was Ellis and Keith.  

Music wise: Undecided: They both suited the settings as L4D(1) music wouldn't suit levels like Hard Rain or Dark Carnival.  

Story wise:  Left 4 Dead 2:  It seems more suitable when the car breaks down or there is an obstacle in there way (Broken down cars or Undrawn bridge) then the survivors getting saved all the time only for something to go wrong which occurred in all four campaigns in the original (Helicopter pilot becomes zombie. 
 
Graphics: Sad to say I couldn't review this since I own the Australian L4D2 where you can't  light zombies on fire, blow out chunks of zombies and dead bodies disappear . 
 
All round fun: For me was Left 4 Dead 2.

The second installation outdid it's predecessor with way more content and the same, awesome, zombie action. I did like the setting/characters slightly more int he original, but I still give Left Dead 2 the edge because it kept me coming back a lot more.

The first game. I find the characters easy to relate to and the game doesn't feel as bloated as its sequel. The new infected and weapons are nice but I didn't feel that any of them were particularly special or exciting to play/use.

The first one.  I could play it over and over and over.  For some reason not the second though.  Don't like how bright the second one is.  No one seems to have a mic in L4D2, I don't get it.  They should create a mic only lobby.  I hate the new special infected in L4D2, and I hate how many of them there are.  The first focused more on fighting zombie hordes, which was much more fun.  Im hoping they have a no special mutation soon.

I love them both... I still go back to play the first one now and again. It had the better characters and setting by far, but the second game has definite gameplay improvements which just give it the edge I think.

I played original a bunch when it came out, and with only playing a bit of part 2, I gotta say the sequel is better. I just find it alot more fun for some reason.

I enjoy the second one more as far as gameplay goes. There is much more variety, which the first game lacked. The original had much better characters and a more effective setting though.

The one with Louis and Zoey.

Never played more than a bit of 2, but I'd probably lean towards the first. Louis is my homie.

I really like them both but the characters in1 are much better to me.

I like the second one, with all the new weapons and stuff. Also with the latest downloadable content adding back the old characters for two levels. I have found with the ability to use melee weapons and newer weapons to work very well. I probally played the first game alot more though.

I enjoy the first game more. Left 4 Dead 2 had too much Special Infected, and none of the new ones are particularly enjoyable to play as or fight. 
 
First game seemed to have more strategy involved partly due to the small amount of weapons. You had to choose between a fully automatic weapon or a non-fully automatic weapon essentially. L4D2 just has several of every kind so it's not as unique in a way.

I agree with the original poster.  The setting of the first game seems much more suitable.  I like the characters better too.
 
I don't like the lack of melee weapons and some of the special infected in 2 are pretty cool - but overall I think 1 is a more enjoyable experience.

i like them both equal. because they are both pretty much the same

I prefer the original Left 4 Dead myself.
 
I just prefer the darker setting of the original.  The 2nd one was a good game but it was too much daylight for a game like Left 4 Dead imo.  The first one had more suspense because of how majority of the levels were in the nighttime. 
 
Which one do you prefer?