Playing though the open beta of MAG, I found a lot more morons online for the first couple of days than I ever did during the closed beta. Read that carefully. The first couple of days. Eventually the teamkillers and the rambos will simply leave from boredom, and I'm sure many of them showed up to ruin the experience for others in the open beta because they lost nothing to do so (open beta was free, Qore beta cost $30, technically).
While I will personally be getting MAG, one of the reasons is because I've never played the first Mass Effect, and it kinda disappeared from most minds by the time I had a functional Xbox. All I ever heard about was the glitches, and how much better the PC version is, and blah blah blah. So, for those basic reasons (hearing only bad things about the Xbox version by the time I actually had a chance to play), and no demo, I never really bothered playing Mass Effect. In hindsight, I really should play through that, and then get Mass Effect 2 later.
Regardless, this is up to you. If you have the money, the question really shouldn't be asked, because the obvious answer is "both." If you don't, we still really can't help you with a good answer. It all depends on what kind of game you like. If you like shooting d00ds and blowing stuffz up, don't really care too much about story, or liked the old SOCOM games and thought they could be bigger, then get MAG. If you prefer having a gripping story and a huge single player experience with lots of places to explore, and are willing to deal with (from what I heard) somewhat wonky shooting controls, then go with Mass Effect 2.
Really, though, how the hell do you have a poll about whether to get one of two ridiculously different games? Even if they were similar games, why have a poll? If I, hypothetically, had a poll on whether to get Dante's Inferno or God of War III, and, hypothetically, everyone and their mother voted for Dante's Inferno, I'd still get God of War III.
EDIT: As an aside, let me respond to some of the things I've already heard about MAG.
"The maps are segmented into several servers and they just tell you that you're playing with 255 other players." The way I heard it actually works is that while a single server is handling everything going on in one map, it doesn't stream everything that happens to the exact detail to your PS3, because that would kill your internet and lag the game like hell. It does however tell you basic information, and the game can render all 255 other players on one screen. Domination maps as a result get crazy near the end.
(The Penny Arcade strip) Really? That happened to me way more often in either Modern Warfare game than in MAG. Especially in CoD4 where people just chuck grenades cross-map.
"No one works together." Not everyone, no, but with the way the chain of command system works, a lot of positive incentive is given to those who follow orders. Anything you do to help with the order that the squad leader gives double XP. Being near your leaders provides nice bonuses to help in combat (a personal favorite, though realism-breaking, is that being near the OIC, who is the leader of your entire 128-man team, heals you).
"More players means more fun." Not... really. It has to be done right. Fortunately, it seems that MAG is on the right path of keeping everything hectic, but still controlled to the point where it's not impossible to know what's going on. The red chevrons and the "danger ring" help this. Speaking of...
(Player feedback) There's been a lot of complaints about the red chevrons in the beta. For those not in the know, what happens is that in addition to the red dot that appears on the radar when someone fires (a la Call of Duty), a red triangle will appear over the head of the enemy who fired at you. While I personally don't mind it that much, since it serves as an additional method of preventing campers, the fact that the chevron appears through smoke is questionable.
What is however nice is the way the "danger ring" works in this game. If enemy fire is heard by your character from any direction, a white section of the ring shows up. I believe it gets brighter when there's more fire coming from that direction, but don't quote me on that. If the enemy fire is going in your general direction, the ring section is yellow instead. Red, of course, means that you're enjoying a nice bullet snack from that direction. I like it partially because it helps prevent the "can't tell where enemies are until you get shot" problem.