The difference is LCD panel type
I prefer the 20'' simply because I dont need the 24'' and the high end 20 has everything I need, but I am hearing about the screens not being as good. Can anyone shed light on exactly what this is about?
The AL iMac 20'' uses a TN panel (type of panel found on most laptops)
The AL imac 24'' uses an H-IPS panel
The white 20'' iMac uses an S-IPS panel, so if you want to go 20'' and LCD panel quality is essential to your needs, you can consider that as an option.
Here are some specific characteristics of each type:
TN film (Twisted Nematic)
- low manufacturing/retail costs
- restrictive viewing angles
- fast pixel response times
- dead pixels display white. Stuck pixels display RGB colors
- lower contrast levels means blacks are not as dark as VA based panels
- lower color reproduction
IPS (In Plane Switching)
- improved viewing angles over TN
- very good color reproduction
- slower pixel response times than TN
- dead pixels display black
- lower contrast levels means blacks are not as dark as VA based panels
Super-IPS (S-IPS)
- same as IPS except ...
- likely best color reproduction of all TFT
- less expensive to produce than IPS
- improved pixel response
Find more info here:
http://www.hardforum.com/showthread.php?t=1039222
or
http://www.tftcentral.co.uk/