GTX 660 is better and will have more headroom to OC than the Boost. BUT, the 650 Ti Boost is nice for the money also. Both good buys.
However, I very much disagree with the CPU pairings being offered if this is to be a gaming PC. K-series i5's are awesome gaming processors for overclockers, and great choices once other supporting components are in check. GTX 680 or SLI 660, then you'd want this kind of CPU power.
Again it depends on your games and budget. But i5-3570K + 650 Ti Boost is not a fine tuned gaming machine, it's nice overall machine also capable of decent (casual) gaming through 1920x1080. If it's strictly a gaming PC, and you play the latest tiles at very high settings, funds should be shifted towards more GPU.