performance wise GTX660 is better. but GTX750 Ti can be OCed to match GTX660 although i don't recommend you got 750 Ti for cheaper and expect you can match GTX660 performance through overclocking since overclocking is never a guarantee thing. also it depends on your PSU unit as well before deciding to get either one. remember GTX660 will need 1 6 pin power connector from the power supply.
for me i prefer a GTX 750 ti , it uses Maxwell architecture and 660 uses Kepler architecture
Maxwell architecture gpu has 135% performance per core higher than a Kepler architecture Gpu . Besides Maxwell run cooler than Kepler . When more cooler = more space for overlocking = more performance . Moreover they will more concentrate on the newer cards driver for performance.