Nikon typically has been as good or better than Canon in DSLR AF, so given the time since their first mirrorless offering, I'm very optimistic that this camera's AF will be on the same level or better than Sony's a1 effort. It may "only" do 20fps, but if it's able to do so consistently without all the caveats required to get 30fps with the Sony, that will be more than satisfactory for me.
I'm a bit concerned about the size. I like the pro-integrated-grip body, but I don't want anything as big as the D6. It strikes me that Canon has hit the Goldilocks size with the R3. It's not too small and definitely not too big. If the Nikon is close, it could be a big winner.
Hmm, consider most rumor sites think that Canon is almost ready to announce, a Nov-Dec release date provides little incentive to move up the Canon announcement. I suspect Canon will announce when the camera is ready and there is a sufficient supply regardless of what Nikon does.