After using the 12wx for some time now, I’m just in love with it. At first, I wanted the larger screen, but after using it, you realize how nice it is having everything on the smaller display. It’s a perspective thing. It looks really small. But when you’re drawing and sculpting, your face is so close to it that if it were much bigger, you would have to look around to see the whole thing. Since it’s higher res than the 21, more fits. It’s great having easy access to everything without having to turn my head or reach for it. The lcd arm I mentioned above enables rotation/tilt, and since I can pull it right up to me, it seems more ergonomic than the big bro as well.
I personally wouldn’t want a Cintiq any bigger than 15". If they could pull off a one wire led display with no parallax, they would be gods.