Brink, easy. But of course this is a Brink forum, so......
However, as someone who has played MAG a good deal, I can assure you with 100% confidence that Brink will be the superior game. Unless you just absolutely can't stand matches with fewer than 64 players, that is. But aside from player count, Brink beats MAG in every way. Teamwork, graphics, movement, customization, balance, coolness, etc.
Graphics and customization, no doubt, but movement is dependant on which kind of movement you prefer, balance and teamwork we're still waiting for the game to prove, and coolness, well that's just opinion

Of course we should note that MAG is a year older, and was targeting the massive scale warfare as it's main goal (causing many of the graphical/customization short comings).