MAG.
Battlefield is better if you want all the realistic crap, like bullet drop and all that. It's very strategic , so much so it leaves little room for controller-skill. MAG is a mixture of strategy and player ability, battlefield is a lot more strategy because of how it's set up, with the recoil/bullet drop/SLOW regeneration.
The good thing about MAG however is whilst team work is good, and all that. You're not reliant on it, on battlefield having an unco-operative squad can frequently cause frustration, and create an overall inconsistent playing experience. It just doesn't play that well in pubs.
Squads of 4 are also stupid. No viable spawns most of the time. Most the time it's a choice between stupid sniper A, B, or C. So yeah, LOOOONG walks on battlefield. Not on MAG.