I'd say the only big question is would you like to use Parallels or Boot Camp?
If not, the Powermac is an awesome computer and can be upgraded for a long time to come. The sheer amount of RAM the Powermac can hold will last you a long time, not too mention the Powermac is built like a Tank! It's very well engineered & designed. It's also physically easier to upgrade. What else...hmmm, it's easier & cheaper to add more storage.
As for the Mini, I think it's an awesome computer but you are sort of 'limited' in my mind to upgradability, the only way I'd pick the Mini is for using Windows/Parallels. That said, with Windows Vista coming out, I think the Mini will be strained to run Vista's eye candy because of the on-board graphics. Either way you can't go wrong, but my vote goes to the Powermac because it simply beats the mini for future proofing. With the Powermac you get a really powerful videocard, lots of RAM space, easier to throw in more hard drives, a faster processor, more ports/expansion.
All said and done, I'm typing on a dual 2.5Ghz Powermac G5 from last March waiting until WWDC so I can buy an Intel 'Powermac' because I have a very strong need to use a few Windows programs. But this machine is still blazing fast for me, so that dual 2.3 machine looks good.
Hope this helps?😕 🙂