In fact, if for Mojave, I prefer HD7950 more than GTX680 now.
The biggest reason is back in late High Sierra, some GTX680 users suffer from the "black box bug" no matter with Apple driver or Nvidia web driver (some text shows as black box randomly). Since it's driver version independent, that means the source of the bug is from something else (e.g. other OS core component).
https://forums.macrumors.com/threads/mac-pro-5-1-odd-finder-graphical-glitch.2130187/
So far, I didn't heard anyone has similar issue in Mojave. However, Apple already shows that it has no interest in supporting any Nvidia GPU. It's driver quality is doubtful for GTX680. And the user may suffer some similar issue in future Mojave release.
So, if price is similar, performance is similar, go for the HD7950 seems make more sense at this moment. Especially if the user only want some basic functions (e.g able to display properly, can shows boot screen, etc).