They use the same GK106 Kepler, they have the same amount of CUDA cores, and their references both have 2GDDR5.
Is the 670 just a 660Ti that's Factory clocked higher by Nvidia to have a moderate between the 680 and 660ti?
More aboutwhat difference 660ti
There is actually a big difference.
The GTX 670 has a 256-bit memory interface, while the GTX 660 ti only has a 192-bit. The GTX 660 ti's 192-bit bus is very crippling and not good for future-proof. Also, you won't be able to game on a higher resolution monitor and it will suffer in high AA. This is why you don't see many people recommending the GTX 660 ti when you could go for a GTX 670 or the similar priced HD 7950
Yup... There is a difference but its not just of the bus interface and it is not effected by bus interface. Its not like that 192 bit is weaker than 256 bit. Its just the width of the bus, thats it.