R9 270X especially the sapphire toxic edition, it gets really close to GTX 760's performance, and since gtx 760 is already faster tha gtx 660ti in most gaming scenarios, I would suggest the R9 270x.
If the price different is $40, is r9 270x worth it?
So the 660 ti is $40 cheaper? If so get the 660 ti - its a great card and with some overclocking (especially of the vram) it can out perform the 760 as it uses the same gk104 but with more cores.
R9 270X and even its smaller sibling R9 270 beat the crap out of GTX 660, you cannot go wrong with the 270/270X, but still as I've said before, get the Sapphire toxic edition, its costlier than the other R9's but boy is it worth it,