I need to find both side by side, for some reason Microcenter didn't have a 21.5 out so I could not compare them. They had a 27 out.
Unfortunately I didn't take pics of it, but I've seen the 20 and 21.5 side by side, alongside a 24 and 27 at one of my local Best Buys. The 20 and 21.5 were just about the same in size, and the 24 & 27 were virtually the same as each other as well. So if you've ever seen a 20 next to a 24 in the past, you've pretty much already seen the same thing as a 21.5 versus a 24.
Well, when I saw them all side by side, I knew I wouldn't even consider the smaller size when the day comes that I'm ready to buy an iMac for home. At least in my mind, getting or keeping a larger size would be the way to go. If the specs of the 21.5 don't blow your 24 out of the water, I wouldn't even suggest making the switch.
I don't have a real picture but I built a composite in photoshop for. Hope it helps. (I threw a few extra models in there out of curiosity)