I have a 21UX and am currently debating whether to upgrade. It seems that:
The 21UX has a more ergonomic art pen (my tool of choice).
The 21UX has a sharper display with better anti-glare.
The 22HD has better colors.
Other than that, there apparently isn't much difference. I don't care about the widescreen, LED backlight being cooler or the 2048 sensitivity.
What I do care about, though, is service. How much longer will my 21UX, manufactured in 2007 last before it gives up, and will Wacom be able to repair it?
Art should tell a story. Don't paint a moment, paint a lifetime.