The two aren't really in the same price category. The 7850 is a $185 card (w/ free FC 3 Blood Dragon, Bioshock & Tomb Raider); the 650 Ti is a $150 card (w/ $75 Game Coupon). At $185 price category, the 660 would be the 7850 equivalent and between those two, I'd take the 660 in a heartbeat as even THG has it well above the 7850. However, the game bundle may be enough to sway you one way or the other.http://www.tomshardware.com/reviews/geforce-gtx-650-ti-benchmark-gk106,3318-19.html
If ya can score a7850 for $150, then between the two you listed, I'd grab the 7850. However, with the 650 Ti Boost on the scene, I'd take that over the 7850.
Even in THG's limited game test suite, the 650 Ti Boost takes a slight 2% lead over the 7850http://www.tomshardware.com/reviews/geforce-gtx-650-ti-boost-gk106-benchmark,3463-11.html
If ya wanna know what other people are choosing look here:http://www.tomshardware.com/reviews/geforce-gtx-650-ti-benchmark-gk106,3318-19.html
The 7850 is AMD's biggest seller from their current generation line w/ 1.63% of the DX11 GFX cards hitting steam servers (20th place) being the 7850.
nVidia has 4 current generation cards in the Top 20
The 670 comes in 11th place w/ 2.25% share
The 660 comes in 14th place w/ 1.77% share
The 660 Ti comes in 17th place w/ 1.72% share
The 680 comes in 18th place w/ 1.71% share
Overall AMD had 5 cards in the Top 20, nVidia has 15
Another consideration might be whether you think you will do SLI or CF in the future. If that's in the picture, I'd read these:http://www.anandtech.com/show/6857/amd-stuttering-issues-driver-roadmap-fraps/6http://www.tomshardware.com/reviews/graphics-card-benchmarking-frame-rate,3466.htmlhttp://www.pcper.com/reviews/Graphics-Cards/Frame-Rating-Part-3-First-Results-New-GPU-Performance-Tools?page=2#comments