You underestimate the power of both fanboyism, brand familiarity, and personal preference (as well as needs). I agree the 670's overpriced, but people still buy them all the time.
It matters for your power bill and nothing else. Efficiency has nothing to do with how much the power supply can output, only how much it needs to draw to achieve that output. On this topic, a good 500W power supply is more than enough for a 7970, but I'm sure the OP can determine his needs better than we can.
@deiidarax: As others have said, the 7970 is the better of the two cards (for the most part); it's usually priced right around that of a 670, but outclasses its performance in games. That said, you should still look up benchmarks for the games you like to play most and see what you can expect from both.