Comment by bigiain

I don't think it matters all that much. Any specific opinions are going to be more a natter of personal taste.

Get one aimed at rhythm/percussion, one that has some bass/lowend sounds, and something to make melodies.

I have a PO12, PO14, and PO16. If I was going to buy another one it'd be a toss up between the PO33 and the PO35. If I were starting from scratch I think I'd still get the PO14, I might get a PO24 or PO32 instead of the PO12, and I might get a PO28 instead of the 16. Or if said young middle schooler has access to sounds they can sample easily enough (like, say, Garageband on an iPad_ maybe jump straight to one of the sampling ones (PO33 or 35) for the lead/melody sounds.