Both of them have their own issues, but historically, Nvidia have been considerably worse, because they not only required a proprietary driver, but also adamantly refused to support certain features, crippling the functionality of a lot of compositors.

Today, I'm having zero issues with an Nvidia card, on Kwin/Wayland. Everything that runs in native Wayland runs flawlessly. Games through Xwayland run great too, now that explicit sync is actually there.

Worth noting that I don't have a VRR display, and don't have a card that supports frame generation. The latter just is not implemented at all, and the I've heard there are issues with the former