I have both 700c (1984 specialized expedition) and 26 inch touring bikes (1992 Roberts Roughstuff and new TT silkroad).
I also have two 650B bikes which I have toured on (Bleriot, kogswell PR)

I prefer the 26 ride, because the tire volumes are larger, generally, than what you can fit on a 700c touring bike. I run panaracer paselas or high end schwalbes on all three sizes FWIW. Bigger (volume) is better as far as I'm concerned. The 26 inch bikes are stiffer frames (smaller triangles) with stiffer wheels. Over the years ive decided that for touring loads I prefer a STIFF frame with larger volume tires to absorb bumps, rather than a noodly bike with high pressure low volume wheels.

My favorite, is 650B with a 400g grand bois 42mm tire. That combo feels fastest and smoothest by a texas mile... wouldnt take it to Mexico though.