We faced this same dilemma a few months ago when we finally decided to update our old G3 iMac. We settled on the 20". A number of reasons; we used the price difference to upgrade RAM, HD and to add the iWork software package (which we like just fine). The 20" also fit into our work space better than the 24" would have. Going from the 15" to the 20" was an amazing jump for us. We don't miss the 24" as we have plenty of display space for all that we do. Our iMac is blazing fast at everything we use it for including web browsing with Safari. We are still using Tiger currently, as it came loaded on our iMac. But we are planning to upgrade to Leopard in the spring as we plan to load Xp or a Vista version of Windows to run some software that is not available or has better function than the Mac offerings.
I would say that depending on where you are coming from will mean much in determining your level of satisfaction with your new iMac. If you are coming from 5 year old technology, as we were, then the 20" will probably be plenty of computer for you. If however you are already using a wide screen with more recent hardware and processing, then you may require a 24" hotrod to satisfy your jones for an upgrade. Your work demands will also need to be taken into account. We do not have a problem with the reported (and, yes they do exist) visual idiosyncrasies that come with the 20" display. If you move from side to side or up and down, you will see color distortion on the 20" monitor that may not appear on the 24". We, however, have no issues with this oddity as we do not rely on "true color" at every angle as a critical need.
The 20", as is, suits us just fine, thank you very much.
