those 2 are pretty much the same with the gtx 660 winning by 5-10 FPS depending on the game - but if you want to game on ultra 1080p on newer games, get something stronger like a gtx 770 or r9 280x.
but you can game on those medium-high settings at 1080p or high-very high on 720p
imo get the gtx 660 - 660ti or r9 270x