There will be 2 Downloadable Episodes extra for 360, wich are AT LEAST 10 hours of gameplay, rumors tell there will be a small extra map. I think the controller is better from the 360 too, but i have to be honest and the ps3 graphics are better according to various websites, though i don't know how big a difference it is.
Achievements may seem dumb for people that have never experienced the addiction you get from it
.. At every game you can get 1000 point's and you just WANT all of em (check my signature). Last thing is that xbox live is absolutely great, though paying for it can be a big disappointment for some people, I think its worth it though.
Max resolution is also 1080p for 360 now, but i have a samsung LE32R82B wich is HD-Ready so i can only play up to a resolution of 720p (wich is realy sharp though!)
Good look with your decision!