The 660 is the better card, so get that one if you can afford it.
Here a comparison between the two so you know what that $50 gets you.http://www.anandtech.com/bench/Product/680?vs=660
From the looks of those number, I think the $50 is worth it.
Dont buy EVGA cards, reference design cooling runs louder and hotter than custom. Get a card from Gigabyte, MSI or ASUS.