If you play BF3, Crysis 2 etc then go for 660 Ti.
If you play Dirt Showdown, Skyrim etc then go for 7870.
Both cards perform better than the other at several games.
I guess the games which show a AMD logo on startup like Dirt showdown, Sleeping dogs etc favor their AMD GPUs.
Check all the games shown in this review, it will help you understand :-http://www.tomshardware.com/reviews/geforce-gtx-660-ti-benchmark-review,3279.html
And get a 600w PSU if you'll overclock anything.
Or else get a 550w PSU if you'll not.