lol GPU is not only the clock XD.
btw GTX 970 is better from both if you wanna buy a new one.
But GTX 780 is better look here.
More memory 3,072 MB vs 2,048 MB 50% more memory
Higher memory bandwidth 288.4 GB/s vs 224 GB/s Around 30% higher memory bandwidth
Better floating-point performance 3,977 GFLOPS vs 3,213 GFLOPS Around 25% better floating-point performance
Higher texture rate 166.4 GTexel/s vs 134 GTexel/s Around 25% higher texture rate
More render output processors 48 vs 32 16 more render output processors
More texture mapping units 192 vs 128 64 more texture mapping units
Higher pixel rate 41.4 GPixel/s vs 33.5 GPixel/s Around 25% higher pixel rate
More shading units 2,304 vs 1,536 768 more shading units