If you compare a racing 700c setup to an off-road MTB setup, then there is a lot of difference in efficiency on the road.
If you set both up for mixed (road/trail) touring, then the differences are marginal.
I use 700c Continental Top Touring 32mm .It's not speedy, but coveres the miles. A 26" wheel with a 1.5" slick tyre at the same pressure (90psi) would have a similar rolling resistance.
When riding with buddies on roadified MTBs, they cruise at the same pace as I do.