Well. The 750Ti is an entry level, low power card. Hardly considered gaming.
The 780Ti is an extremely high end gaming card, was fastest single GPU out there for a while.
You can see here:http://tpucdn.com/reviews/NVIDIA/GeForce_GTX_980/images/perfrel_1920.gif A comparison of all of them.
The 780Ti pulls a massive performance edge, and the 980 edges out the 780Ti.