Which GPU is better between these ones?

Is a ASUS GTX 780 OC Edition better than a STRIX GTX 970 OC?, or the GTX 970 is better?
I saw the cost of both of them and the GTX 780 was more expensive than the 970? Why?
The GTX 780 has a 384-bit memory interface, Does that influences in something?
And also the number of CUDA cores, The GTX 780 has 2304 CUDA cores meahwhile the GTX 970 only has 1664 CUDA cores, I have 2 questions in this case:
-What are CUDA cores?
-Does the number of CUDA cores influences the FPS while gaming?
So that last question...
Which one should I buy? and Why?

But the 970. Performance will be roughly equivalent, but power usage is way lower and overclocking headroom much higher with the newer card (970).

Go with the 970. If that's not enough, consider two 970s or a 980, or wait for the "Maxwell Titan". Only get a 780 if you can find one dirt cheap.

^Yes.