Resistance is pretty fun. Definitely get Ninja Gaiden Sigma, but only if you don't mind the difficulty of it. Consider getting Stardust on the PSN as well. It's pretty good.
PS3 has been really ramping it up, they've been really aggressively dropping their price, really working on creating some good in-house games, and have been bringing up the quality of their online interaction/play (with the help of 2.41). I would go as far as to say I enjoy my PS3 more than my 360, and I definitely use it more.
Edit: Oh yeah, give a consideration to Uncharted and Ratchet and Clank.