I don't have personal experience with these cards, but according to research that I made, I think the 7970 GHz is a better option.
Pricing: The sapphire 7970 GHz is $470 and the lightning is $50 more at $520 (still not going down strangely!).
Performance: I don't have any personal experience with these, but looking at the clocks, I think the 7970 GHz Should be the slower judging from the clocks. The 7970 GHz clocks is 1000 MHz without boost and 1050 MHz with boost, while the lightning come with 1070 MHz. They share the same cores by the way.
Overclocking: They should come with similar OCing results, the lightning may overclock better but I think the 7970 GHz shouldn't be far behind.
So, as stated earlier if I were to choose between these two cards, I would choose the 7970 GHz. Not that fat behind the lightning but $50 cheaper. Unless 7970 Lightning price drops. But you shouldn't go wrong with the lightning either.
Also, just make sure that you have a good system to go with these 7970s. Otherwise the 7970 is going to be pretty much useless.