Right now one of the biggest controversies in gaming in general are these two games. Modern Warfare 3 and Battle Field Bad Company 3. I personally prefer PS3.
Now, Modern Warfare 3 is suppose to be almost the same game only updated. What is suppose to be the same about MW3 is the graphics. MW3 is going to have the same 60 frames per second game play making it very smooth while playing. On the other hand, BF3 is now a better graphic system than Battle Field Bad Company 2. Battle Field 2 was based on the original DICE Technology Server. BF3 is now going to be based on a newer better Dice Technology Server.
Another huge difference in the games are the multiplayer match length and maps. MW3's matches are noticeably shorter. Battle Field takes the game length and makes it much more realistic by almost doubling the match length of CoD's. The maps in MW3 are going to be smaller than BF3 maps. On BF3, what makes playing on the large maps fun is that you can drive vehicles such as helicopters, atvs or 4 wheelers, tanks, and small UAV's on online multiplayer matches. On MW3 online multiplayer matches you cannot drive these vehicles. Another factor that will be disappointing to CoD fans is that in BF3 you can't get kill streak rewards. As for BF fans MW3 has kill streak rewards. In case you do not no what that is, it's when you get a consecutive number of kills in a row without dieing. You get rewards such as attack helicopters and such.
So if you were wondering about these 2 games, I hope this helped! I'm not even sure which game I'm going to get! Check back here soon for more PS3 Gaming updates. Thanks!