If you see the great praise Mass Effect 2 is getting right now, you'd pretty much assume that ME2 would clearly be the better quality game with more sales. I think that's a no brainer.
However I do not mean that MAG is crap or something. From what I've been hearing, it's very solid and for a game that supports 256 players with no lag with pretty solid graphics, that's a big achievement on its own.
So basically both games are on different levels of gaming in which both games have their merits and frustrations (refering to MAG) which I think will be enjoyed by the people who buy the games and both games will be loved.