The 21" is more powerful (700W push, 1000W SP, vs 600W for the 20"), has a wider range of cutting heights, has three height positions for the handle instead of two, offers a SP version, and has a variable speed blade motor to adjust the blade speed to cutting conditions.
I have the 20" and I'm happy with it. Some people with extremely thick grass and/or grass that is cut at a very low height have commented that the 20" can lack power, requiring a slower walking pace to keep from overloading the mower, but I don't find I have this problem. However, I generally cut my grass at the highest setting.
One advantage the 20"'has over the 21" is weight; the 21" weighs more, but even the SP version weighs considerably less than a gas mower.