The 20” aluminum iMac has a crappy LCD
https://marco.org/2007/12/18/the-20-aluminum-imac-has-a-crappy-lcd
Do a quick review of Wikipedia’s guide to LCD types.
- The 20” has a TN panel. This is the older, cheaper LCD type that looks brighter at the top than the bottom, and the brightness changes quickly as you move your head up or down. In addition to the horrible vertical brightness gradient, colors look awful and black levels are terrible. Many people are returning their 20” iMacs because it’s so bad and so noticeable.
- The 24” has an H-IPS panel, the newest and best type of LCD available today. It’s one of the finest LCDs money can buy.
Looks like you’re FAR better off spending the extra $300 for the 24”. In addition to the massive screen real estate, the quality difference is huge.