Love both, own both. Dead Space 2 hands down. Better story, awesome gameplay, a nice survival/horror feel. But if you just want a solid FPS with gorgeous visuals then Rage is great, story & ending sucks.
I own both, and I really didn't like Rage. I'm into shooters, but this game wasn't fun for me. I've started and stopped playing it a few times now. There's just nothing that makes me want to keep playing. The story isn't very compelling and the combat
is lacking in my opinion.
Dead Space 2, however, has a good story and is definitely fun to play.