Palit GTX 780 Ti Jetforce vs Gygabite Windforce 780 Ti

both are good but if you want to overclock really high I would go with the windforce strictly because it is 8+2 phase design vs 8 for the palit. the cooler on the palit is very similar to the windforce as well but from the factory the palit is overclocked a little higher
 
Define efficiency.

And... why do you care?

Any GTX780ti will perform well and really needs no overclocking to do so.
Both cards use fan type coolers which cool the gpu chip very well. But, they dump the heat into your case where case cooling must get rid of it. That heats up the cpu cooling air, not good.

I prefer the stock titan type blower coolers. They get the waste heat directly out the back of the case.
Here is an example: http://www.evga.com/products/Product.aspx?pn=03G-P4-2883-KR

As to overclocking, the vendors are wise to this and use the better chips in their factory overclocked versions. At probably a fair price premium.