I don't know about those two cards in Premiere, but in AE the GPU makes very little difference at all.
That being said, if you're using third-party plugins for AE that use the GPU (like Element, for example), it's a different story. For Element, I've heard the GTX 970 is the best bang for your buck; going up to the 980 wouldn't be worth the difference in price.
Thank you, you help me a lot... Anyway, I presume that, a better card will be fasterrendering with Adobe Encoder... thanks
I'd ask about the choice of cards on the Premiere Pro forum. It can actually benefit from a good card. AE? Not much.