ThanatosFX :
The GTX 750 ti uses a lot less power and it's newer and cheaper however the GTX 660's performance is better but the gap is not that big nonetheless.
Additional info:
You can not SLI the GTX 750 ti, but the 660 you can.
GTX 750 ti has more Overclock potential since its power consumption is quite impressive.
It's nothing amazing. I maxed out my 750 Ti at 1410Mhz, and the boost wasn't that breathtaking that I'd feel the need to run it 24/7 at those speeds. But then again, this was my experience with it.
I had the MSI Gaming one, no PCI-E psu connector.