Well, to be honest, both of those games are in pretty different genres. Crysis 2 is a shooter while Mass Effect 3 is mainly an RPG (but it does have fundamental shooter elements).
Personally, I'd say that Mass Effect 3 is incredibly better than Crysis 2. I really thought Crysis 2 was a "meh" kind of game. But if you don't care about going a little bit older, I would suggest Mass Effect 2, as I enjoyed that game way more than the third. :P