I have a similar dilemma.
This weekend I'm planning on trading in my Power Mac G5 and my Dell 2005 fpw (along with my beloved 17" flat screen CRT ADC) for an iMac. I'm either going to get the 20" G5, or the new 17" core duo. The only thing I'm going to be doing on this computer is running logic 7 express, and basic iLife stuff (and the occasional movie). I'm going to bite the bullet and do it all at a reseller instead of eBaying it and buying the computer from Apple. It's a bitter pill to swallow as it means I'll lose out on my education discount that would knock the price of th 17" to $1400 CAD and the 20" to $1620 (instead of $1500 and $1750) and get less money for the computer, but I don't really have much of a choice.
I still haven't decided which computer I'm going with, although I have been leaning to the G5. I'll try them both out and try and get a feel for whether the 17" screen is large enough, because in all honesty, I can't see there being a significant performance difference in anything other than rendering/encoding video.
Correct me if I'm wrong, but performance between a 2.1 ghz G5 and a 1.8ghz Core Duo is pretty similar, right?
Obviously I'm trying to spend as little money here as possible (especially since I'll have to add RAM), but moreso I'm trying to gage which one is the best value. I'll be holding on to the computer for a little over three years and then getting a laptop, whereupon the computer is either going for sale or to my parents (replace their 15" iMac G4).