The 7870 its cheaper, and at worst performs the same as the 660 TI and at best performs better. (Benchmarks from different sites have been mixed)
Tom's says they "think it performs 5% + or - from the 660 TI". They can't seem to make up their mind even though they have benches done that show a consistent 7870 advantage. Unfortunately the Senior editor of Tom's, seems to have a bit of an Anti-AMD bias. He can't help himself.
This HD 7950 looks like a great deal. Depends if u can afford it and/or get it shipped to your location. Another thing u should take into account is that you ll prolly need a decent 550W power supply for a 7950. A 7950 overclocked comes ahead when compared to a stock GTX 670. You should really consider it.