Average performance wise a 660 Ti is about 10+% faster max overclock vs overclock and if based on that, pricing should be about $20-$30 more than a 7870. I refreshed my memory and the >20% advantage depends on certain games - but for that kind of advantage the $50 is worth it. Buying a 660 brings features like GPU PhysX of course.
There are a number of factors that that went into my decision to go with 660 Ti, beyond wishing to spend money or pro green bias:
1. Future proofing - yes yes, nothing is future proof. That said, a 7870 capable of 1200+/~1600 core/memory is just enough to max all games these days @ 1080p. However, even though there may be no groundbreaking technologies around the corner, future games will undoubtedly be even more demanding, and that +10% or greater advantage that the 660 Ti has will mean higher framerates and higher settings and may even extend the useful lifespan of the card, let us say a year. That's totally worth $50 in my books. Of course, this depends on the games I'll be playing, which brings us to the next point,
2. Battlefield 3, and Total War. I really like Battlefield 3. That +10% advantage that the 660 Ti has could mean the difference between 45 fps and 50 fps in a Battlefield like game - between choppy and smooth in that engine.
I'll definitely play Rome 2: Total War; maybe lots of it. The 7870 struggles in the Total War engine and here is where the 660 Ti can have a 20+% advantage.
3. Overclocking. I have a Galaxy 660 Ti 2gb GC with 6+8 pin power connectors, and with other features it is built for overclocking. I can't say for other people but I was pretty sure it could overclock like a beast and lo and behold it does. The 660 Ti's in general seem to be good overclockers like the 7950s, whereas the 7870 is hit and miss.
The MSI Hawk 7870 with its GPU reactor and unlocked voltages is likely to achieve 1200+/~1600, and perhaps the XFX Black Edition, and I would get a Hawk if I were to buy a 7870, with the caveat that the blue LEDs might keep me up at night. But considering that it's only just enough to max the games I want to play, it just doesn't have enough juice going forward. I want to have a card that's more powerful, the 660 Ti is the next step up and the cheapest, and I'll pay the $20 premium ($50-$30).