The big advantage of 26" wheels is that tires are easier to find. Every Wal-mart, K-mart, and bike shop in the U.S. will carry 26" tires. 700c tires are more difficult to find, especially wide 700c touring tires.

Toe overlap is less of a problem with 26" wheels.

For on-road touring, you probably have slightly more frame and tire choices if you go with 700c wheels.