In fact, since I’ve had some experience with some of the Smith boards, here’s my very brief breakdown of the ones I know something about:
Prophet 6 - for vintage sound, can’t be beat. So thick, that multitracking requies enough scuplting to take some of its charm away. A typical band or solo synth. Lacking in modulation, which makes it more a player’s board, less a designer’s.
OB-6 - same as Prophet 6, but with the Oberheim character. Tougher and rawer in character.
Prophet 12 - a sound designer’s dream. Both the module and the board is extremely well thought out. Can sound like anything, except for hands down, truly thick analogue. The difference might not matter in most contexts, but where it does, it shows. Most of the time, who cares? It can go to places a pure VCO synth never can.
Rev2 - in character, a lovely in between of the Prophet 12 and the Prophet 6. In terms of sound design and interface, it’s fantastic. If you know your way around subtractive, you’ll learn this one in five minutes. Amazing modulation capacity, combined with a pretty clever gate sequencer. The poly sequencer ain’t bad either.
Prophet X - not sure what to make of this one. Quality is superb. Samples mixed with classic oscillators is a cool concept. In reality, though, I found this one came closer to boards like Kronos and Motif, just because. The filters compensate, of course. But in terms of why I should ever get this one, if I could ever afford it (which I never will), I haven’t found a reason yet.
Tempest - the odd guy out. So funky, so cool, so flexible. Yet, anything you do with it requires more labour than usually should be necessary. Granted, it’s groovy that you can abuse parameters where others have put a lid on and created safety zones to find sweet spots faster. But it’s cooler in theory than in practice.