The reason for using the 6 bolt disk pattern is there are a large number of cheap disk hubs. It can be hard to find 135mm fixed hubs for mtbs. Other than that the traditional fixed hub is superior, cogs are available and cheap and easier to change.

I would use something similar to the shimano centre lock or cassette. Splines and a single nut are a easier to change than 6 little bolts that tend to round out and seize.