I have to go for neither. Or at least, I don't think we'll see enough of a bump to be meaningful or clearly from these games. Mass Effect 2, for all the hype, is a hybrid RPG (although argubably getting lighter on the RPG and heavier on the combat with each iteration) that I can't see catching on with a broader audience beyond those who bought and enjoyed the first.
While MAG is new, and is positioning itself to offer something new (massive online battles) again I think most people who would want it already have a PS3.
GT5 I expect to shift consoles, not these two.
Try to be reasonable... its easier than you think...