Just ask yourself a simple question: Is narrative important to you?
Bad Company 2 is a fantastic game. It's a well realised modern warfare skirmish game- tense, unforgiving, carnal fun... and that's all it is. There are a lot of people who can throw themselves at this experience all day, every day for weeks or months. If you're one of them, this is for you.
Mass Effect 2 is a brilliant RPG, easily the best of Bioware's current bunch provided you're receptive to the sci-fi setting. It has a superb story, a steady stream of new content being released (not for free mind you), and it warrants repeated plays which absorb about 30 hours a pop- you'll be playing for a month solid before you've seen most of the content and outcomes, and you'll probably be happy about the necessity of this investment.
These are two VERY different ideas. Surely, you must feel a leaning more toward one than the other? =) Good luck.