Running AMD GPU’s will be a lot less hassle (& the open source drivers are quite good) - nvidia drivers require a bit more maintenance & the nouveau open source drivers do not work with hardened kernels.
If you spend some time making KVM GPU Passthrough work the performance will be good enough. I’m still running an FX 8370 with an RX 570 8gb passed through to the Windows VM & play everything on high / ultra @ 1080p. Using real-time fifoschedulers in libvirt for the pinned cores & iothreads is the secret sauce see my previous posts for details.
I have Steam working in Linux & proton installed but it is overall less hassle to just game with the Windows VM (I think Wendell came to the same conclusion). My system is also dual boot but I stopped booting into Windows around 10 months ago. With 2 screens I can game on the Windows VM on one & monitor Linux stuff on the other.
Another thing to consider is AMD driver support lasts a lot longer than Nvidia.
If you start with 2 GPU’s & a dual boot system you can experiment with everything. Rx 560 4GB works well as a host card for Linux.