Which iMac?

I just bought the 21.5" for economical reasons. but as Helhammar says, go to the store and try it out. Even if i had the money i think i would be hesitant since the 27" is huge, but i only use my comp for daily stuff and word processing.

 

I agree with Hellhammer you may be best off going to a store and seeing which screen size you prefer, between these two models especially as the deal breaker between them is the screen size.

I just bought a 21.5 actually. When in the store the 27 inch was amazing, the 21.5 was too, equally so, but looked tiny in comparison when side by side. I think this is more due to the 27 inch being truly gargantuan in size though rather than the 21.5 being tiny which is deceptive. The 21.5 looked much bigger when i got it home

 

I preferred the older 24"
It was just right.

But being the choices as they are, I'd personally go 27".

 

If you need the screen real estate then the 27" is they way to go for the price.
If not, the 21.5".