Using TomsHardware figures, the R9 280x consumes 255W, while the 7950 consumes 188W. Let's take something a bit less than the top figure from the mining comparison page, and you get, say, 780 kh/s for the R9 280x, and 700 for the 7950

700/188 = 3.72 kh/s/W
780/255 = 3.05 kh/s/W

So, if you accept the performance selections I chose, the 7950 is still the superior card (by a fair margin, I'd say). Of course, there are other factors, such as the limited number of cards you can get in a rig, and of course, the price. The R9 280x is a bit pricier, but it seems most people are shifting to the R9 280x because of the extra hashes per rig.