Some touring bikes are now marketed as more "go anywhere" bikes. They have mountain bike wheels rather than 700C roady wheels.
The answer for us may be given by the fact that we have entirely 700C bikes already. Ancient Family Wisdom has been that 700C is better on tarmac and possible unpaved Sustrans tracks. 26 is slower, but stronger.
What is the real difference between the two? Also what effect on the ride of the bike and geometry? Lower I guess. Shorter?
I currently have no plans to go to the sorts of places where 700C spares would not be available.