Well late in the game here, I am now regretting the purchase of the 21.5" only because I cannot use this machine as a display in target mode. Now that I have this knowledge, it may definitely shape which iMac I possibly buy to replace this 2010 21.5". It would be nice to get another couple of years out of this as a display, but, hey, hindsight right?
I've managed with the screen real estate just fine.
I think I'm realizing that 27" iMacs are perhaps for advanced users and 21.5" are for average users who want to Facebook. I could be wrong.