I'm using the 21.
My suggestion is to decide for yourself whether you want portability or not. With the 12, if you have a laptop and don't mind the mess of cables, you can take it anywhere where there's a power outlet.
Not so much with the 20, but you have a bigger drawing or painting space. More express keys, bigger res, etc.
Or get the two if you can afford it and also have a video card that allows for dual displays. Set the bigger one as the primary monitor, the smaller one to contain your tool palettes.