If it was me making this purchase, I would try to stretch and get the 27 inch iMac. It's upgradeable, has the most up to date hardware, and a larger screen (when I was deciding, the 21.5 was nice but the 27 is better, especially for photo work!)
The 27" is a nice machine.
I really wish that the iMacs came with SSD standard like the MacBook Pro and air...

