Which graphics card is better?

I would recommend something else, definitely. you could get the base model of the same cards that have decent cooling, OC them yourself and save a ton.

between the 2 you mentioned the 7850 is much better once both cards are OC'd. (and by much I mean a lot). And even then you could flash a 7870 bios onto it and pull even more juice out of it...

but if you're at a 1920*1080 resolution and looking for cheap in gaming, you can always go with a 7790 (or a 650TI boost if you find one cheap enough) but not anywhere lower then those.

tje 660 try upgrade to a 660 ti on ebay you can get them cheap