I don't know guys, I am so confused!
Get neither of the two options as they are both a waste of $ as their performance advantage over GTX970/390 in GTA V is very small despite the massive price premium.
What you should do instead is get an after-market GTX970 or R9 390, drop MSAA down from 4X to 2XMSAA and overclock both cards. You'll get your desired performance and save $140-150 towards a 16nm HBM2 GPU in 2016-2017. If you spend 40-50% more for a 980/R9 390X over a GTX970/390 for GTA V, you would have just wasted $140-150 for performance difference you won't be able to tell without doing actual benchmarking. That $140-150 is way better used to get a card 50-75% faster in 2-2.5 years when you can just resell the GTX970/R9 390 and use the $ saved from not spending it on the 980/390X + the resale value from your 970/390 card towards a next generation card with 8GB+ HBM2, more modern features, better DX12 support, etc.
An R9 390 ~ R9 290X and GTX970 OC can easily gain 10-15%+ with OCing as well.
Besides, in outdoor areas with a lot of grass, even GTX980TI struggles to maintain full 60 fps with MSAA in GTA V so really based on the nature of the game, you won't achieve fully locked 60 fps with AA at all times in GTA V. What you are looking for is good performance and GTX970/R9 390 (or 290X) deliver all of that for a lot less $ than the R9 390X/980.
Gigabyte Windforce R9 390 =
$300
EVGA GTX970 SSC =
$310
PowerColor R9 390 8GB =
$310
And if you really want to get the best bang-for-the-buck, you can get an after-market 290 for $230, overclock it 10%, and then by end of next year, sell this card, take $170-220 you saved from not getting a 980/390X + resale value on a 290 and get yourself a $400 Pascal/Arctic Islands GPU. In GTA V, GTX980 is just 15% faster than an R9 290 but it costs 90-95% more. You do the math. R9 290/290X/390/GTX970 are all solid choices imo.