I went with 265/65-17.
There was no compelling reason to go one way over the other, but I felt the 265/65-17 spare will fit with just a bit more clearance. Some of the things that go into the receiver hitch are a bit long and an extra half inch may prove valuable at some point.
You're right -- the Revos seem to be on backorder in most places. Tirerack has them in only one warehouse last time I checked, with recovery at the other warehouses scheduled for next week. After coming up dry in their first searches, the local shop found 4 of the 5 tires I wanted and got them in overnight; obviously we took those since you can always change out the spare later. So that I can go to a 5-tire rotation scheme, I'm ditching the last AT20 and putting all Revos on.