No, there is no clear cut winner in efficiency. It depends entirely on which card and what resolution. However, I'm going to have to give it to AMD because the 7750 is so ludicrously efficient as far as performance per watt it dominates most resolutions.
its an endless cycle, each one attempts to beat the other succeeds until the other makes a new line.
for instance, when amd created the HD 7k line, it was more efficient than the gtx 5xx line. then nvidia comes out with the gtx 6xx line which in general is more efficient than the hd 7xxx line on the high end(low end wise, the Radeon is still more power consumption effective, as the HD 7750 is generally better than the gtx 640)