I've ridden a rigid framed MTB with 26" wheels and a similar one with 700c. They were both from the same era with similar geometries but the 26" bike was suspension corrected while the 700c wasn't/isn't.

I feel really tall on the 700c MTB.

There are lots of skinny tire options for 700c wheels but I don't know that either size has a clear advantage. There are those that say that bigger wheels will smooth out the bumps more.

The MTB with skinny road tires on it is surprisingly quick. It's not much slower than my roadie but if you're going to ride on skinny tires, you might as well ride a road bike.