The GTX 650 Ti Boost seems to be slightly better on average. But they're so close that which card yields higher fps depends upon the game title. Some games are optimized more for AMD cards, others are optimized more for Nvidia cards. So check performance in the titles you play most often.
http://www.techpowerup.com/reviews/Zotac/GeForce_GTX_650_Ti_Boost/9.html
http://www.techspot.com/review/661-nvidia-geforce-gtx-650-ti-boost-sli/page5.html
*** Update ***
If you can spend a little more for a GeForce GTX 660 or Radeon HD 7870, it would be worth it. The GTX 660 is down to $200 before rebate, and you can find the HD 7870 for about $215.
Note that results from HWCompare don't accurately reflect real-world performance. Their results are just theoretical estimates (i.e. educated guesses) based upon the manufacturer's published specs, not based upon any actual testing of the cards. In many cases they're never even physically seen the cards in question! Their theoretical performance (which is clearly stated in the disclaimer) should be taken with a grain of salt. Always look for actual in-game test results from sites like Anandtech, Techspot, Guru3D, Techpowerup, Bit-Tech, Tom's Hardware and Xbitlabs.
Here's the real difference between a Radeon HD 7850 and 7870:
http://www.anandtech.com/bench/Product/549?vs=548