Whats the difference between these two 7970s?

Well there are a few differences.

1.) Higher grade components. The Vapor-x cards have black diamond chokes, and caps. Which in turn results in how the card handles power delivery.

2.) Cooling performance will be stronger with the Vapor-X it uses a Vapor chamber to exhaust heat.

3.) Also the Vapor-X is a ghz card which is a binned chip it basically means that yes its a overclocked chip but also its the fastest chip that AMD currently offers at least fastest single chip.

SO it comes down to do you value the premium ghz card or are you just fine with the regular 7970.

Would the Vapor-x have room to overclock more than what they provide?

That I'm not quite sure of. From what I've gathered from what little I know of the ghz cards is that they don't have as much overclock headroom as the normal 7970's may have. I look at the ghz edition and feel its the card to get if you want the best without overclocking.