I don't know how it relates to the 700C wheel size, but the Ribbon of Road guys went from Alaska to Argentina on Co-Motion Americano bikes that had 700C wheels. Apparently the bikes themselves did well, but the wheels not so much:

"Rear Rim - Cracked the first time in British Columbia, replaced in Seattle, cracked again in Mexico, replaced in Guatemala. Cracked again in the Bolivian desert, replaced in Argentina. Current status of rim 4 - cracked on the Carretera Austral in Chile, glued and duct-taped together. Keepinī my fingers crossed! Thank you Co-Motion Cycles for sending replacements!"

Scroll down on http://www.ribbonofroad.com/journal.html

I would imagine a smaller diameter wheel might be stronger that a larger one, all other things being equal, but I have to admit that 700C bikes do "feel" faster on the road - but that could be due to other factors such as gearing, rim size/weight, tire weight and size etc etc.

If I was building a bike for all-over touring (as I am) then I would use 26" wheels. To me it kind of comes down to a choice between "better road bike" or "bike like jeep".

Edit: Another factor that should come into play here is bike fit. For shorter people, overall height and body dimensions can make it difficult to make the frame small enough to be able to accommodate the larger 700C wheels without compromising the geometry. Smaller people often would be better off with 26" wheels, since the frame can be much more reasonably proportioned. Also, you're less likely to have issues like toe overlap with the front wheel with 26" wheels, and incidentally there's more space on the downtube for the underside full-size water bottle, and even sometimes extra stuff behind the seat tube (if the bike has long chainstays). You can still build a 26" wheel bike for taller people, there's nothing stopping you there, but building a 700C bike for a small person can be tricky. The most important thing is fit - if that doesn't work then everything else is out the window.

Finally, often the 26" wheel bikes have larger tire clearances, and there is a better range of larger tires and knobblies available for 26". But at the same time, you can still put smaller slicks on there if you want more speed on the road, so I think 26" is perhaps a bit more flexible in the options.

Neil