Jazzman 1945 is our other professional melodica teacher.
I have all three of those Suzukis, Balázs, and very much prefer the M-32C and A-34C over the M-37C. The M-32C and A-34C are very similar to each other in terms of playability and sound. There’s not much to choose between those two except for the range of notes. I have played them both quite a lot for the past several months. The M-37C is also an excellent instrument, but I find it clumsier to hold, and it does not have as clean a sound as the other two. It is also a bit harder to disassemble and reassemble because of the way the melodica fits into its metal tray; the configuration is the same, but the fit is tighter, so that more prying is needed against the tray, especially when putting it back together. For some people, though, having the full 37 keys would be a reason to prefer the M-37C. I like 32 keys and prefer starting at F rather than at G, so I like the M-32C best. The unique key range of the A-34C is cool, though, and makes it special.
I’ve mentioned this before, but on all three, the screws in the end pieces engage tiny metal inserts, which is a good thing since they don’t screw into plastic; but it’s easy to have one fall out and be lost if you’re not careful.