I would probably also go for the 760, just because it has newer technology and is less likley to be faced out in the future.
Regarding the 760 being the new version of the 660, my guess is that it's more a representation of price. The 760 is a price with similar price to the 660 but with newer technology. It's probably more aparent with the 770 and 780s. The 770 is a 680 with increaced memory bandwidth while the 780 is a Titan with lower memory bandwidth. Pricewise they are the 670 and 680s replacements and they are better than the 6-series counterparts performance wise.
In the case of the 760, I think the 660 has a sligthly faster GPU but the 760 has faster memory.
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