Given equal bikes, like the Bruce Gordon, the Atlantis or the Heron Touring, which are available in both wheel sizes, there isn't that much. Still:
- A 26" wheel is a bit stronger. A 36-spoke 700c wheel compares roughly to a 34-spoke 26" wheel... which means that a 32-spoke 26" wheel is weaker, but a 36-spoke 26" wheel is stronger than a 36-spoke 700c wheel.
- 26" tires are available all around the world... but I have also read that the 26" format may be different in India or Viet Nam from what we use here. IOW, not really a factor in North America or in Europe, but it might be in other countries.
- 26" tires are available in different widths. However, anything narrower than 26" x 1.5" might be really hard to find on the road. Fat tires, MTB style, are easy to find, however.
- 700c tires are available in different widths up to 47 or even 55 mm. However, you might have a hard time to find anything wider than 28 mm in small shops, and 32 or 37 mm in most LBS... so if most of your rides are on gravel or mud roads, 26" might be the best, but if most of them are on good packed gravel, stone dust or asphalt, than 700c might be better.
Wheel diametre vs mileage
St. John's Street Cycles in U.K., at http://www.sjscycles.com,
has a whole article on the benefits of 26" tires. Not sure that I buy all of that. Still, overall wheel diametre also depends on tire size, and if you adapt gearing to actual tire size, wheel size doesn't really matter.