I would go with the 660, you get a better price/performance ratio. You can also play most modern games at high/ultra settings, with the 650 TI you would likely be hovering in the medium/high range. What games do you plan on playing, (if any) also would your PSU be able to power a 660?
+1 to that. The GTX660 is more than worth the price difference. As you've been told, the performance difference is huge (not quite double though). The GTX660 will max anything at 1080p and AA except Far Cry 3. GTX650 Ti can still max a lot of games, but the more demanding ones will need you to drop settings or disable AA. GTX650 Ti is a cool little card and not a bad purchase, but the GTX660 is a far better buy. You really will get a lot extra for your money.