I think you should decide based on the bike, not just wheel size. Does one frame feel more flexy when loaded with camping gear? Does one hold a straight line better when you are day dreaming and not concentrating on the road? Does one handle better loaded if you suddenly have to steer around a pothole or obstruction? Does one feel more solid going down a hill fast with a load? Does one have wider gearing with the ultra low gears you will want to climb steep hills? Does one have a geometry that would feel better for a long day in the saddle? If one is drop bar and the other flat bar, if so which do you prefer? Does one have more reliable components?

Regarding 26 inch tires, I actually like a narrower tire for pavement than 2.0 width. I used Schwalbe Marathon 559X40 (or 26X1.5) on a trip in summer 2012 and they worked great. I do not recall the pressure rating but I think I was running about 80 psig rear, 65 or 70 psig front. The specific tires were the plain Marathon with Green Guard adn wired bead, I mention this because Schwalbe makes so many tires with the word Marathon. The bike in the photo below has these 40mm wide 26 inch tires.



My point on tires is that you can buy a 26 inch tire that will hold the higher pressure and roll as well as a 700c tire that is 35mm or 37mm wide. But you won't find these in stores along the way, so if you might want to use tires like these, start the trip with them instead of trying to upgrade later.

The other tire in the photo, I am not sure what size tire that is but it looks kind of heavy.