There are so many types of Mylers, and I feel that they are well made, very nice bits that eliminate the nutcracker action of normal jointed mouthpieces but still allow you with lots of options.
Remembering of course the level system. The idea behind that is to bump the horse up and get more tongue relief through the levels. Level 1 comfort snaffles are excellent for starting a colt in. Level 2 is a little more tongue relief and more palate pressure. Level 3 is for experienced horses, maximum tongue relief, and lots more palate pressure.
Really, just find what your horse likes. I am currently going to be showing my colt one handed in an S shank Level 2, my mare shows in a short shank level 2, the colt trains in a twisted Myler D ring level 1 and we have several other smooth level snaffles for training. I love the customizeable option a lot.