650ti and 650ti boost are different 650ti boost is like 13k and 660 is around 14-15k. So for comparison 650ti is good BUT since 750Ti is there so you might wanna grab it since it is just 10% - 15% slower than 650Ti boost and wont be requiring any larger power supply so you can just get it.
For 650Ti boost vs 660. i would say 650Ti boost.
650ti is totaly different and slow then both.
While 750Ti is better than 650ti but slower than 650Ti boost / 660