While idling, the GeForce GTX 680 and Radeon HD 7950 backed by their respective binary drivers were each idling in the low 40s (°).
The overall system power consumption was not terribly different while idling with both of the proprietary drivers supporting proper dynamic clocking based upon load.
The GeForce GTX 680 was about ten frames faster than the stock Radeon HD 7950, but both graphics cards were running at more than 100 FPS.
But when breaking down the Xonotic findings to performance-per-Watt, the Radeon HD 7950 (at stock and overclocked speeds) was both noticeably better than the stock GTX 680.
The GeForce GTX 680 also ended up running significantly warmer than the Radeon HD 7950.
The GTX 680 was burning through about 37% more power than the Radeon HD 7950 at stock speeds.