I got my 21 in the mail today and I freaking love it... so much that I'm sort of meh on the 20 now! Here is a comparison between the 20 and 21.
The 21 is mostly hard plastic with a slightly more flexible hip piece, smooth matte texture throughout. The joints slide easily from pose to pose, but hold position very well. The flexibility of the multi-piece neck is really awesome. 21 comes with optional pieces to bulk up your dude's thighs (not pictured).
There was already a really great video review of the 20, but I'm just going to add a bit here too - the skin texture is really nice, but it makes the seam on the front of the thighs a bit unsightly. I know most people don't care so much about stuff like that since the figures are going to wear clothes, but it kind of bugs me since the rest of the figure is otherwise so well crafted. By comparison to the 21, the 20's joints are much firmer and a bit harder to move. The overall feeling of the 20 was stiffer and a bit less poseable - the range of movement is somewhat limited by the bulk of the muscles (elbow shown at maximum bend).
I have a 22 coming, but I have no idea when it's going to get here.