IMHO, it's not as clear-cut as all that. For starters, the G4 CPU wins at OS X; OTOH you have to use XPostFacto to even install it. The G3 cache in the iMacs is smaller than that in a desktop/tower G3, or in a G4 upgrade - this affects performance, under any OS.
The 7300 can also take faster PCI video cards, and whatever other PCI upgrades take your fancy. Admittedly, adding Firewire and USB takes at least one of your available 3 slots, if you need to match the iMac in terms of those ports.