This is a fascinating question that I hope others will address. I am almost always working on Jazz standards, trying to learn improvisation and playing the same tune in different styles and multiple keys.
I don’t have an opinion about which melodicas are best for Jazz, but I do find myself choosing to use different ones (from my vast collection) in working on different tunes. The current Yamahas more often than not, except for the obscure little yellow Suzuki Study II 25, from which I feel I consistently get the best sound. I’m looking for a woodwind sound and ease in manipulating it in striving for bluesy, jazzy effects.
It may just be me, but I find it easier to get a Jazz sound out of a smaller melodica, especially from a 25-27 key model. My guess is that I’d feel even more this way playing blues, where the smaller number of keys would pose fewer limitations.