The 280x uses slightly less power for the same performance. The 280x is basically a better binned 7970ghz. The 7970ghz should have a game bundle with it so I would probably lean towards that. Even if you're into those games, you can sell off the codes.
Could you link the specific cards plz?
Here are the power consumption figureshttp://www.techpowerup.com/reviews/MSI/R9_280X_Gaming/24.html