I agree totally about the re-supply issues, but I wouldn't be surprised if the the 650s fit with v brakes. It can even be an advantage in that the higher block position changes the cable travel and may make the use of travel agents unecesarry. Still this kind of thinking vanishes on the road, where the big issues are reliability and resupply.
I think the 26" is the better choice on touring bikes, if one takes the mountain bikes out there, whether beaters or thorns, and the high end touring bikes like Sakkits or Gordons, there is a lot pushing the 26" tire on touring bikes. But even if that isn't true, on smaller frames it is a no-brainer.
Another issue on 650s might be clearance for fenders if you are using them.