With regard to rim diameter only. The 26 inch mountain bike size tire is 559 mm bead seat rim diameter. The 700C racing bike, touring bike, etc. is 622 mm bead seat rim diameter. These are pretty much the common current wheel sizes on new bikes in the USA and Europe. Not sure about the rest of the world. Recumbents and folding bikes, etc. may use other wheels.
There are a multitude of tires available almost everywhere for either size, 26 or 700C. Anything from 1" to 2" can be found for both rim sizes. But 26 is usually easier to find in more places. Wal-Mart sells 26 tires. But they do not sell 700C tires. I'm not advocating you buy your tires at Wal-Mart but if you are in a town that does not have a bike shop and you need a tire, the 26 will be much easier to find. I also think the 26 mountain bike craze has infested the entire planet more than the 700C racing/touring bike has. So in deepest, darkest Asia or Africa, you are more likely to find 26 size tires instead of 700C.
As for wheel strength, 26 is stronger than 700C. Enough to matter? Not for well built touring wheels with 36 spokes and durable rims. Either is strong enough.
If I were ordering one of the Bruce Gordon bikes and had the choice of 26 or 700C, as he provides, I would choose the 26. Mainly for the easier tire finding ability everywhere in the world. And hopefully his bikes are built so they can take up to 2" wide 26 tires so you can use the same bike with knobby tires on the worst roads of South America, Asia, Africe and with 1.25" slicks on paved roads in the USA/Europe. With a 700C size, you probably do not have the clearance for extremely wide 2" knobby tires for the worst roads. So you would have to use the 700C touring bike for tours on paved roads and the 26 mountain bike for touring on unpaved roads. Having two different touring bikes is not a bad thing.
