Hi Ken, I have ukes with 12", 14" and 16" inch radius. Although I can't tell a huge difference in feel between them my two ukes with 16" radius are the easiest to play. So based on that I would pick a 16" because I have never tried a 20".
I have a theory based on my experience that because of the narrow width of a ukes fretboards they are better served with a shallower radius, at least for my hands. The #1 string falls away from the plane too much on a 12" and a lot of times the A string is not played cleanly with barre chords. This is what happens on my two 12" radius ukes. Again this is for me with my hands and my technique...........or lack of it