Duittuit,
I think it might depend on you and how much you want to mess with it.
If I understand it right - the GTX660 is stock and not overclocked, the GTX660ti comes from the factory already overclocked.
Arvee enjoys overclocking so he would probably spend less money and do the overclocking himself - while I on the other hand, would rather have it come already overclocked.
Sun :-)