The GTX 660 is the stronger card but they are close.
The 660 benches at 4113 and the 750ti at 3816 according to passmark:
http://www.videocardbenchmark.net/gpu.php?gpu=GeForce+GTX+660&id=2152
http://www.videocardbenchmark.net/gpu.php?gpu=GeForce+GTX+750+Ti&id=2815
The 750ti is rated as the best value in terms of performance vs cost at a ratio of 25.44:
http://www.videocardbenchmark.net/gpu_value.html
So, if you can get your hands on a GTX 660 on sale there's no question as it is the stronger card but if you're just comparing dollars to performance then the 750ti is the way to go. Keep in mind, the 750ti does NOT have sli capability while the 660 does so for future upgradeability if you plan on or like the idea of at least being able to go sli then keep that in mind. You'll have fun with both but for the price difference of less than $20, I say get the 660. I think you'll have more fun both short and long term. Just my 2cents! Hope this helped!
Thanks,
Justin S.