The 660 Is ahead of the 750ti in terms of performance but the 750ti Is still one of the best budget options as i have owned one and loved it. I'd be taking the 750ti As from what i know they are probably more reliable and a lot more have been tested in the past but if you have the resources and stuff then go ahead for the 660 but still consider the 750ti.
Watch a few videos on the 750ti and the 660 to get an idea on it