It amazes me how many people want help with a game but don't even have the common sense to state the title. That would be like me walking up to some one asking "how do i go there" where am i going? No one knows :/
Personally I've played both and i was really hyped for BF3 and not looking forward to MW3 in the slightest and i hate to admit is but i'm enjoying MW3 more. BF3 is still a great game but it has it's problems, i haven't been a CoD "fan boy" since 4 i usually just play them for the story but i'm actually enjoying it online and off. To me it feels like CoD4 almost, kinda like what MW2 should have been. They are two completely different games tho; BF3 is huge maps with vehicles and MW3 is close quarters maps with lost of running and gunning. Both are fantastic games tho but i'm leaning more towards MW3, i hope BF3 is a little more playable by the time Back to Karkand comes out this December then what it is now tho.