21,5" VS 27" iMac

If you get the 27" iMac you can upgrade the memory yourself for about ½ of what Apple charges. Look at MacSales.com and Crucial.com for memory upgrade packages for a 27" iMac. For $100 at MacSales you can get two 16GBs modules to take your memory up to 24 GB which should be more than enough for your image work. I have 16 GB and it's more than enough for Photoshop and all else that I do.

I'd also recommend getting an SSD drive, as big as the budget will afford, instead of a Fusion or rotational HD.

The ram and the chip matter little. You'll have ample power either way. The deciding factor must be the screen, and there is no competition - the 27" is miles better.

You can in the 27" models. Not so the 21.5".

NOTE: Make that two 8 GB modules for a total of 24 GB and $100. ;