I'm doing a research for my own analysis, so I'd very much appreciate if you folks could poll and comment 
I'd always get a 27" without a doubt - that beautiful big canvas is perfect for everything I do and I find the 21.5" far too cramped.
I love my 21"; it's large enough for everything I do, but small enough that it doesn't dominate everything else on my desk. It nestles in quite well between my 20" ACD and 20" iMac G5![]()
It's pretty much neck and neck. I've always had the 27" but have friends with the 21 and they seem happy...I have become very used to the real estate, and it's my main desktop.
Nothing wrong with the 21" though, and your poll seems to confirm that...about fifty fifty.
For people who have more typical needs said:That's my choice! Real estate is limited on my desk.
27" all the way - unless footprint is an issue, there is absolutely no reason not to get the bigger screen.
I have bad eyesight. You would think the 27" would be a no-brainer. It's not. I have the 21 inch iMac and the 27 inch Thunderbolt Display. I like the 21 inch screen better. Too much physical maneuvering needed with a huge screen. Of course the glare and bluish tint compared with the 2012 iMac screen might also be a factor. They definitely upgraded the screens last year.