Well, I'd go with what you are familiar with and are use to riding. As far as availability, well after reading hundreds of Crazyguy journals, I can't remember but once or twice that someone had a massive tire/wheel failure. And while Wally World, etc. will likely have 26 inch tires, in this day and age, who can just get on the Internet and order a 700 tire and have it overnighted to whereever you are (I'm talking US). A well built 26 and a well built 700 will both be more than strong enough. Most mountain bikes now use 700/29er wheels, and those of us riding offroad aren't experiencing tire/wheel failures anymore then when riding 26in.