27" 2560x1440 IPS is one of the most technologically impressive displays thats ever been widely available to consumers. Apple and a few other companies used to push those 30" 2560x1200 displays but with better image quality and higher pixel density, the 27" IPS panel is arguably better.
With my education pricing, the higher 21.5" SKU is $1400 and the lower 27" SKU is $1600. The iMacs seem to be identical besides the screen size. For $200, I would be very tempted to get that larger screen.
I don't know what you mean by "casual gaming", but not even the highest GPU available in the 27" model will be fast enough to play modern games at greater than the 1920x1080 resolution of the smaller monitor.