I personally would go with the 750 ti over the 650 ti, or look at a 770. but it all depends on your budget.
I too would get a 750Ti over a 650Ti,the capabilities of those two are so close that the 750's lower power consumption makes it a winner for me.
But this is a 650 Ti Boost
,it has a bigger bus(192bit vs the non-boosts 128bit),which takes the Memory bandwidth from 80gb/s to 144gb/s. Thats a large increase and when you consider the 650Ti Boost is stronger than the 750Ti in other areas (Pixel and Texture fill rates),that makes the 650 Ti Boost a clear choice of the two.