Well it depends on what you like. Modern Warfare 3 Multiplayer gives you that fast gameplay that makes it interesting, while Battlefield 3 Multiplayer is more open area and if your not good at Battlefield 3 it can be quite difficult to get kills with everything going on. But Battlefield has more "Team" based multiplayer and you get more of a realistic experience, which is one of the reasons Battlefield 3 is one of my favorite games right next to Modern Warfare 3. They both give you TOTALLY different expirences and is hard to say wich is better because in reallality they are very diffent games.
No it will not. But they will have a survival mode in the spec ops menu. Which will be somewhat like zombies except it wont have and windows or anything and you wont have to purchase areas. it will just be an open map and you have to survive ass long as you can with an infinate number of rounds and the rounds go from Easy to Veteran diffuctly the longer you last.