the stock WTB 1.5" tires are absolutely huge, and ride like it too. for my transamerica trip i'll probably get a 1" or 1.25" tire. I really like the 1.5" tires for off road touring and commuting/ bike paths etc due to the way they handle bridge joints and the like, but i cant help but think of the energy i'd save by slimming down just a bit. I have a feeling the WTBs run wide for 1.5" tires, too.

edit: back on topic. i don't think you can say that either wheel size is qualitatively better. Ideally, you could have your choice for any size. In the states atleast, mtb tires and parts are easier to come by. I've never really been abroad so i can't really speak to overseas availability. I have, however, been to PR and Mexico, and there are mountain bikes everywhere. working at a shop, i know we always had a pretty good selection of 26" mtb/cruiser tires in stock for the dry rotted rusting away huffy beaters than came through the service center, and there were times at which road tire selection ran thin (especially in sizes suitable for touring). Bottom line: size by the top tube and it'll be hard to go wrong. Don't let the wheel choice influence you too much, because if the bike doesn't fit you everything else is just gonna be an after thought. see my bike in all of its load-hauling glory in the bike porn sticky thread. its only got a few hundred miles on it.