the thing I find funny about all these wheel size arguments, is that the first production "mountain bike" was originally intended to run a 700C wheel with big volume knobbly tire (what we now call 29'er)
the originators tested both 700 x 2.1" (29'er) and 26" x 1.95" and actually preferred the performance benefits of the larger wheel / tire combo.
but for a supply problem with the 700C knobbly tire (in contrast, Schwinn had a warehouse full of 'Beach Cruiser' 26" knobblies), what we have always called the "mountain bike" with its legacy 26" wheels, would have actually been a 29'er from day one of mass production
I spent most days since 1986 riding 26" wheels on the dirt, and since getting a 29'er last Winter, have not looked back or ever missed the smaller size
once you learn to ride the larger wheel properly (not a test ride, or jumping on a mate's 29'er for a lap) the benefits become very noticeable, and for me its all about being able to go faster whilst expending less effort
I can turn the dirt trail into a pump track more effectively with the bigger wheel
Schwinn with their "beach cruiser" tire in 26" provided the business confidence for the originators of the production mountain bike to go ahead with their ugly duckling