RobCrezz :
1) probably the 970 due to more vram
2) 970 consumes less power, with similar performance (probably higher with overclocking)
3) 970 better with overclocking.
What less power? They both consume ~350W when under load what's a lot of power already.
And performance wise there is a massive difference 980 has everything unleashed and the gigabyte model has more efficient cooling that the 970 which uses a more tamed version of the 980s chip.
And frame wise for 1080p around 10-15 frames faster and heaps faster for 4K.