Don't get me wrong Gigabyte GTX 970 G1 where fantastic cards, I love them, power density is awesome, they are extremely stable, beautiful (i know, I'm a nerd, can't help it) but when it comes down to power efficient, it's like 40% more than 750Ti (including the rig overhead!).
On AMD you probobly undervolted your cards to get them to use less power. On Nvidia you change the TDP of the card. You can do it in software with the nvidia-smi tool, or modify the bios. (voids warranty)