The mouth pieces of both are comfortable, though different. On the P-37D, the spit valve arrangement does not include a direct vent out of the melodica, so moisture vents into the outer casing, from which it drains or evaporates. But this does not create odor or result in clogging. As a matter of fact, the moisture does not have to be blown out as often as is the case with many other melodicas.
I personally prefer playing the Yamahas without a mouthpiece, as Jon Batiste typically plays his melodicas. Doing so is not more comfortable, but it provides more of a feeling of intimacy with the melodica, more feeling of the vibration, a kind of sensation that you might like in the absence of the visual aesthetic. Because of the nature of the mouthpiece opening, playing without a mouthpiece is trickier on the metal tray Suzukis, though Jon does it. There is a space between the plastic mouthpiece receptacle and the plastic encasement of it in the melodica endpiece – I’m not describing this very well! So for them I like to use the fitting end of the long tube as a mouthpiece, or another more minimalistic alternative mouthpiece. This is, of course, a matter of personal preference. The mouthpiece of the Yamahas has a duckbill shape, so that the opening is a slot rather than a round hole. It feels good in the mouth; but, in my opinion, a round opening allows for more control, for example in bending notes. If you lived near Boise, Idaho, I could let you try out my melodicas and mouthpieces.