I would choose the Powermac for a few reasons.
1. I want to have my PC and my Mac using the same display. The Powermac makes this easy. With the iMac, I would need a second display.
2. I will set up the Powermac in Raid 0 thus giving me much faster disk access. Not a big deal but since the hard drive is often the systems bottleneck, it can make a noticable difference. Not to mention that the Rev A iMac G5 had poor SATA performance. Perhaps it has been improved?
3. As more programs become multiprocessor aware, the Dual will become more of an asset. Also, OSX will take advantage of the second processor in non MP aware applications during multitasking. That can't hurt.
4. I bought a 9800XT that was purchased as a BTO option from Apple from a guy on eBay. This will nearly double the graphics power of the Powermac as compared to the 9600 in the iMac (according to barefeats tests involving the two cards).
5. I do not currently have need of PCI expansion, but it would be nice if it was available.
6. 16x Superdrive vs 8x slot loading Superdrive. I would personally rather have a tray but I know that there are probably just as many people who like the slot loading drive better.
By the way, the setup I have decided on includes the previous generation PM DP 2.0GHz along with a Dell 1905FP.
A few things that made this decision more difficult were:
1. I hate cables. The iMac is so elegant and minimalistic and the Powermac pales in comparision in this regard. Although, there is something to be said for a giant brushed metal beast under your desk.
2. The iMac is such a great deal, especially the 20"er with 2.0GHz G5 and 9600 with that gorgeous screen all for $1699 after student discount and $1599 if you want to wait for a refurb.
3. Did I mention that its damn sexy?
