I'd be willing to bet that the two Mylers Al described in fact ARE the most-utilized among Miniature horses. They are the only two I have, also. I move my beginning driving horses into the comfort snaffle after the basic eggbutt when I feel they're ready. The 'mullen action' one is the most versatile bit I believe I've ever used, bar none--I have yet to have a horse that didn't like it! Now, some of my horses go equally well, and rarely, a bit better, in another bit(varies from horse to horse)-but it works QUITE well across the board!
I am still on a quest for a bit that my 38" mare truly likes. We have moved closer and closer(have now had her in four different bits-currently, a solid copper mouth w/ low, wide port...she likes the mouthpiece pretty well, but doesn't need the acidity of the solid copper, as she has a wet mouth anyway.) Am considering looking at the new 'Myler-like' ones, too-or, may go to a two ring butterfly if I can find a good one in the right width(3 3/4")with a suitable mouthpiece.
Margo