Technically, both do the exact same thing - they're hanging-V drawbots, with a bit of poking, I'm sure you could compile the polargraph firmware to run on a Makelangelo hardware - which is nothing more than a RUMBA, a board initially developed for 3d printers (actually based on RAMPS). It's an excellent choice for a platform. The Polarshield (which is the "unique part" of the PolargraphSD) has the same motor drivers, but a different kind of LCD, and that's about it really.

The Makelangelo has seen more iterative development on it, and there's been lots of different versions with more significant changes, whereas Polargraph is much more of a side-project for me so doesn't get much TLC - especially in the last year or so.

Polargraph software is better looking, but probably less feature-rich (no plug in architecture, for instance). It's fatally limited by being a Processing app rather than a "proper" java app, but on the upside, it means its a little easier to setup and hack in (somewhat).

Polargraph has always been an art project first, and an engineering one second, whereas Dan (Makelangelo's designer) has always been pretty up front about the educational value of his machines, how they can be used as a tool to learn about trig and programming and CNC and maths. Dan is actually an engineer too, so that probably helps.

I'm interested in the way the Polargraph machine expresses itself, the foibles and habits it has, the way it's good in some areas, and it's bad in others - it's not just a plotter. I'm interested in showing off it's weird internal coordinates system, instead of hiding it behind a facade of cartesian g-code, that's one of the reasons the machine has so much built-in behaviour (like the pixel shading patterns), and the control protocol is more "opinionated" to favour built-in features. Most other machines like this are designed to be as abstracted and as plotter-like as possible.

Historically, Polargraph was first of the two, with Hektor being the inspiration for it, and the AS220 drawbot firmware giving me a head start in the implementation. Der Kritzler happened at about the same time, so there was obviously something drawbotty in the air at the time.

Your final point about Makelangelo actually being available is probably the most pertinent - Even if you did think the Polargraph was best, there aren't any to buy! I will be making another batch of Polargraphs, but who knows when.