Just look at the memory speed of each card, and the core speed of each Gpu.
The other thing to look at is if both cards have the same amount of shadier cores.
Some cards had the full 2048 cores while other had less to keep the price down a bit.
One more thing is the memory size of each card, most 7970 cards come with 3gb or 2gb of memory.
So go for the one with the higher amount of memory fitted to the card if the case ok.