I haven't played either games but both look solid. MMA appears to be more realistic but UFC seems to offer more content. Which do you guys think is better?

EA Sports MMA seemed incredibly impressive. Watching the TNT, the fights themselves were kind of exciting to watch whereas UFC just looks like a repurposed SmackDown! game.

if you are like me and love to just play games like this for the online.  There really is no question. 
 
EA MMA online actually works, for one.  Being able to watch replays of your last 5 fights is a nice touch (as well as everyone on your friend's list).  I enjoy having a good fight and then just sitting back and watching the replay and noticing all the things I didn't notice while I was too busy playing.  One fight I took a knee to the face and my nose started slightly trickling blood.  Punch after punch and my nose started dripping more and more blood.  By the end of the fight my whole front of my face was covered in a streak of blood (it was one of those 25 minutes 1 round fights).  Every punch I took drops of blood would fly out and land on the canvas and remain there....awesome details like that are fun to see....didn't even notice when I was actually playing.
 
Fighter share is also nice.  The CAF system is lacking imo.  You can only use photogameface or pre-rendered heads.  No adjusting sliders which is disappointing if you are wanting to make...you know...UFC fighters.  Some people have made some decent ones tho.  
 
Also, the way the belts are set up is nice.  You rank up quickly and are put into contender fights were you and another guy that has contender status will fight for a belt.  Then you can win the belt and defend it.  There are multiple belts flying around so there won't just be one guy with the belt cherry picking opponents so he doesn't lose it.  Speaking of cherry picking, you can't see anyone's win/loss ratio before a fight starts which is nice...so people backing out of fights hasn't been a huge problem tho it still happens occasionally because you can see people's belts (white belt, yellow belt, orange, green, blue, etc.).  
 
Another thing that makes the online enjoyable is that all the fighters are pretty balanced with the exception of a few.  So you never feel like you are at an extreme disadvantage if you don't pick one of the fighters that is suppose to be considered the best in that weight class in real life.  Like Fedor has pretty ridiculous stats....but that's understandable.  
 
So, for me, without even a second's thought, EA MMA >>>>>>>>>>>>UFC Undisputed.  And this was just me talking about online.  Career mode is also leaps and bounds better and doesn't feel as much as a grind as Undisputed's does.  It's sad that few will give this game a chance and I can bet it won't sell well.  Hope EA doesn't drop the franchise.

I played the demo of MMA, and I didn't like it very much.

In my opinion MMA looks the best and most realistic, but I think the gameplay of UFC 2009 is FAR superior to MMA and UFC 2010.

EA MMA in my opinion is the much better game, although both are fun. I had UFC 2009, 2010, and now EA MMA. Both UFC's have been sold back but I think I'll keep EA MMA for the long haul. The career mode has been much more enjoyable as it wasn't as much of a chore to do the training. Gameplay wise I really like the control setup of EA MMA. The way way the standup, ciinch, and grappling control similarly is much more seamless to me. The standup is much less of a rock em sock em robot battle and seems more tactical, I think the use of the Fight Night engine here is really good. The submissions actually work and the ground transitions are much smoother (the use of rumble to signal transition attempts is really good). No dumb button mashing or shining here, it's all controlled motions. Overall on the gameplay, I didn't feel like I was fighting the controls near as much as the UFC games. 
 
Variety wise, this game has more diversity as the training locales and the actual fight locales are very unique as well as the different fight rules. Also, the characters seem more unique from each other. The only downside is the lack of UFC fighters. Even though I know a lot of the non-UFC fighters it is a little anti-climatic in career when Strikeforce is the top level and it's like the UFC doesn't exist. In career I'm in the lightweight division which is actually really good outside the UFC so it's not bad at all (Heavyweight is also another weight division where it is probably just as good as the UFC's).  The roster in EA MMA is really lacking in the welterweight & middleweight divisions. I would say that the presentation is more mature here. You definitely get the feeling of MMA being a global sport which is pretty cool. 
 
As a long time MMA fan, this game feels much better to me. It's not perfect but this is a big step forward in my opinion. Now that EA is in the game I think that THQ really needs to raise the level of their game considerably. UFC 09 to 10 was not a big jump IMO (actually liked playing 09 more). Unfortunately I don't think they'll be able to compete quality-wise, especially since EA can share tech between this and Fight Night and they have much more experience doing quality sport simulations.

I've only just got the EA game and had a couple of fights, so this really is a first impression more than anything. I love the fluidity of the fighting, it feels great to play and it is much more techical in terms of timing your stirkes and transitions. I have been playing the UFC games since they came out and i still think the tranistion system in that is better. (partly due to the fact i havent got the timing down yet in EA and im still really new to the game) but the stirking and the submissions are spot on. i'm hoping to get some online games under my belt at some point today once i get a handle on the controls, but damn if i am not imprssed with this game at the moment.
 
my only gripe with it at the moment and it is mostly me being new to the controls is that tranistion doesnt work with me as of yet but i am hoping running throught the careea will help out with that, as the tutorial is pretty bad.