Mass Effect is a sure thing, you know it will be good, might not be great, but will definitely not suck.
As for MAG I am not really sure, I've played the beta and didn't find it that great, but then again we were limited to small teams and couldn't experience the 128vs128 matches.
I would just wait 2-3 days after launch of both and check some reviews, and in case they get similar values or appreciation, get MAG, at least its something new, not just another game to bridge the first and third games of a trilogy.