Modern warfare 3= Attention deficit disorder taylor made game, it is the michael bay of video games.
Battlefield 3= successor to the military fps ever aka battlefield 2, it is the hurt locker of video games.
if you want action setpiece after action setpiece thrown at you, with no real substance or atmosphere, get modern warfare. its good. if you want a slower paced game that pulls you in and lets you fight out on massive scale and detail (including better animations, much more realistic sounds from weapons and atmosphere, graphic engine and destructible environments), get battlefield 3. different games are different. im getting battlefield 3, activision can kiss my ass, no modern warfare for me.
Also, note that everything of BF3 shown at e3 was PC game footage, not console game footage. so it will probably look worse(it will) on the consoles as pc's technology is years ahead, but it should be well optimized for the xbox/ps3 so it wont look bad either way.