ME2 will have a bigger opening by nature. Its a RPG. After that the sales will drop drastically, as all RPG sales do. MAG will have longer legs, although I doubt it'll outsell ME2 lifetime, even thus.
Off topic, MAG could be a DLC monster for Sony in the future. I would wager that MAG, as a money making platform, will make more money than ME2 ever will.
No online-only game has ever sold much more than 1M (look at Warhawk, the latest SOCOM, etc.), so if MAG sells 2M, that'll qualify as a landslide success, especially if it has word-of-mouth and DLC legs. ME2 would have to beat ME1 to qualify as a major success, in my book. I think that's possible, but usually sequels have bigger openers, and lesser legs, than their predecessors, and the earlier games end up on top, lifetime.
These two games are in radically different genres. Their sales profiles will be similarly different. Its not easy to compare them, except in obscure ways -- sequels to smash RPGs have bigger openers than online-only shooters, and that's pretty much all there is to go on. Kinda pointless to compare them in that way, since financially, their models are also much different.
