Blizzard is a contributor of the Khronos group (Vulkan), and it's of their most interest to bring lower-level APIs to WoW to mitigate the CPU bottleneck problem. They'll port their engine as soon as they can.
If we're talking about low-end cards, then sure go with Nvidia. When we put those ridiculously powerful cards to run WoW, then it doesn't rly matter at all.
Look at this if you're not convinced:
But going with AMD you'll have better performance down the road, even if right now it doesn't matter.