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I am about to buy an iMac. 27" or 21"...?
Get the one that best meets your needs, preferences and budget. Only you can determine that, as this is very much a subjective decision. What works extremely well for someone else may not work well for you, and vice versa.
 
Also consider the space where it will be used. Will a 27" fit in the space? I have a 23" ACD on my MBP and have issues with my bifocals. A 27" would be worse.
 
Get the one that best meets your needs, preferences and budget. Only you can determine that, as this is very much a subjective decision. What works extremely well for someone else may not work well for you, and vice versa.

Exactly, unless you give us more to go on there is no way anyone can give you specific advice. All you will end up getting is people justifying their own purchases.
 
It may also depend on your eyes (over 40 - wear glasses?). Perversely, I've found the 27-inch more difficult to use than the 21.5-inch, because:

1. The resolution is higher (I think) on the 27-inch which makes text and all the menus etc slightly smaller to read, even though there is more screen real estate.

2. And the amount of LED light coming from the 27-inch - due to the surface area - is increased, which I also found more difficult to look at.

So I can view things better and more clearly on the the 21.5-inch than the 27-inch.

(N.B. this is highly subjective and other people will have different experiences).

Plus, going from a laptop the 21.5-inch is huge anyway.
 
I am about to buy an iMac. 27" or 21"...?

Also, consider if you may want to install more ram, or install more in the future. If you want to install ram in the 21.5 inch model its a big job has the screen and a lot of the components have to be removed to allow you to install the ram.

Whereas on the 27 inch model. There is a little access panel that pops off to allow you to install more ram.
 
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