My P37E arrived today, so I thought I’d add to the discussion with some initial thoughts…
To me, it looks a bit more like a retro melodica than the P37D. Although I like the slightly rough finish of the plastic, I prefer the overall, more rounded shape of its predecessor.
There is considerably more blowing resistance when compared to the P37D. This is fine for me, but I wouldn’t call it an improvement.
The sound is more muted, in the way that the Hammond 44 is more muted than the Hammond 44HP. If you’re recording, or amplifying, you could recreate this on the P37D by rolling off some of the high frequencies on a graphic equaliser, or tone control. In a live setting the muted sound may help you to blend with other instruments.
In brief, it looks a bit different, sounds more muted, and takes a little more effort to play with the extra air resistance (though some players may like this).
In general I prefer the P37E for its sound, but would turn to the P37D if I needed brightness to cut through a mix, or to give me more (pre eq) options while recording.