I run both sizes depending on where I am touring. All my on road touring is primarily done on 700C. If the route includes very rough roads or fire trails, the 26" wheeled bike goes. Some small frame sizes need the 26" wheel size to make it work. I actually think there are more tire selections available in the 700 rather than the 26". Take a look at the type of touring you are planning and choose what works for you.
Aaron