If you run at stock clock speeds the 660 swings between being 15% slower and equal to the 660 ti's power, depending on resolutions and the game being played. At the same time the better gtx 660s will cost you around $230 dollars compared to $300 for the best 660 ti cards. I'd prefer the 660 in terms of pure value since that's 30% more expensive, unless you are buying the whole system right now, in which case the value advantage would probably go to the 660 ti.
The problem is you'd be best off buying a cheaper-end 7870 ghz edition if you're looking for value.
^Agree with kevin, if u really looking for price/performance value, get the 660..it is like 8~10 FPS difference between the non ti and the ti version,,,but the ti version is much more expensive....