If you are going to be doing a lot of buring, you may want the more traditional slot loading burner in the PowerMac -- since it would be easier to upgrade if you so wish.
The slot loaders "can" have trouble with CD/DVDs if they deviate from the norm in thickness and/or size (which is a nasty no-no on the odd size/shape CDs) -- especially when turned on the side -- and are harder to find upgrade replacements for, since you're stuck with the more expensive mechanism.
The SP PowerMac 1.8 is a headless-iMac in design...
So you're swapping a LCD screen for a more traditional burner, upgradeable video card, more memory slots, and a spot for a 2nd drive.
---
If you plan on keeping the machine the PowerMac isn't too bad a deal (since you'll be able to upgrade it and keep it fresh a bit longer), if you plan on selling it to make the upgrade -- it probably won't matter which machine you get.