I'm waiting for the 2012 iMacs, and I'm also going to be playing Diablo 3.
As long as they keep the resolutions the same, I am going to get the 21.5" over the 27". The 27" has way more pixels which means more processing power is needed, unless you run the game in a lower res.
The better video card in the 27" may be able to handle it, but I don't know the exact numbers.