In terms of power management they are about the same; Its the power that is loss that creates heat that nvidia has a problem with ( nvidia has almost but solved that with the 500 series )
As for buying a card, i would wait a couple months if its not needed asap. The 6000 series so far is just the "lower" end cards for the series. The higher end cards are due out at any time which will make the prices of the older cards drop/skyrocket depending on which card you wanted.
When picking cards always think about the future. Do you plan to crossfire in the future?
Also do you dual boot non-windows OS? AMD tends to completely drop the ball when its comes to anything but windows.
Also check out some of nvidia's cards, they have come down in price.