I guess you will probably have done this but nonetheless........When you go into the Apple store try the 27" at the same distance away from the screen as you will be sitting at home. In the stores I find because you stand and are generally further away from the screen than you might be in normal use in a home environment.
I mention this because I have a 27" and if I was buying again would probably settle for the smaller screen. Whilst it isn't too much of a problem but when I sit at it on my desk the screen is VERY big and fills more than my eye can take in.
Both screens are impressive but with hindsight I think I would go for the smaller and save a few quid.
Just a thought before parting with your hard earned.
I have both an older 24" iMac (same 1920-by-1080 resolution as the 21.5" current iMac) and a 27" iMac and the 27" is way better for working with as the resolution is so good that I can have multiple windows from multiple applications running. Often I might have my TV tuner running in a window while working on a document and browsing the internet, all viewable on the one screen at the same time. I work about 20" from the screen and fine it no problem to work with.
It all depends on what you want to use your Mac for. I often using apps that use the full 27" 2560-by-1440, That’s 78 percent more pixels than the 21.5-inch iMac display. I would say that if you can afford the difference then go for it.