It all depends on your demands. As far as I am concered, I would choose the 660 because it playes almost everything in 1080p up to 4xMSAA, and also because if I wanted to spend more money I would choose 7870 or 7950.
Some reviews I have seen have noted that an overclocked 660 can match a stock 660 Ti. I still don't think that justifies spending less on a lower performing card. There is a certain level of performance that can be considered the bare minimum, and I would have to say the 660 is right there just above that level. Of course, the 660 Ti also overclocks and gets up into top-end territory performance when you do.
So the real question is if the GTX 660 is enough for your own intended use. Take a close look at your resolution and the games you intend to play. If you are happy with the performance benchmarks, then don't spend the extra. I personally feel that if you have the extra money, get the most card you can afford.
The performace difference at stock:
"The GeForce GTX 660 is 13 to 16% slower than its Ti cousin at 1920x1080 and 13 to 17% slower at 2560x1440 across all the tests. The difference in speed doesn't seem large considering the 30% difference in price."
The performance difference when the 660 is overclocked:
"With just a couple of exceptions, the GeForce GTX 660 Ti is still in the lead. Other factors being the same, the lack of 28.6% shader processors in the GPU affects the performance of the GeForce GTX 660 so much that the higher clock rates can't make up for that." http://www.xbitlabs.com/articles/graphics/display/gefor...