For me, the P-32 is somewhat easier to play for the following reasons: (1) It takes less air due to the size of the reed chamber. (2) It is easier to know by feel where you are on the keyboard and in general to have a non-visual command of the keyboard (probably less of an issue for a professional pianist, but the feel of a melodica keyboard is very different from that of a piano)). (3) It is easier to move the melodica into position. I never use the hose and am moving the keyboard with my left hand into position in relation to my right hand — this is, of course, related to (1). (4) The P-32 is easier to cradle and balance in the left hand, not only because it’s shorter, but also because it’s narrower and lighter. The weight also affects the demands placed on the left hand. There are some ways of holding (or manipulating) the melodica that are much easier with a 32 or even a 25-27 key model — for example, if you sometimes hold the melodica vertically and seat the bottom end in the palm of your left hand, or if (like Lowboy Bootay) you have special techniques that involve moving the melodica around as you play.
But I don’t think you can go wrong with either one! How much the extra keys of the P-37D matter probably depends somewhat on what kind of music you play and your own style.