One note, just for accuracy's sake, there's a Quicksilver and Quicksilver 2002, and the MDD was originally launched in 2002, there's a 2003 revision with firewire 800 (not compatible with OS 9) and a further revision from 2003 which removes firewire 800 and adds the single 1.25GHz option and adds back in OS 9 compatibility. Sometimes that machine jokingly gets referred to as the "XPress Edition" (because Quark took longer than almost anybody else to build an OS X version of their software.)
The main thing that differentiates the QS'02 and makes it require newer OS install media than the dA/QS (which, again, are AFAIK literally identical) is that some components of the Ethernet interface changed and it requires newer drivers. The technical specs are otherwise basically the same and any dA can be upgraded with QS, QS'02 or third party parts to run at the faster speeds.
Here's the reasons why I'd choose a dA/QS/QS'02 for OS 9 over an MDD:
1) guaranteed ability to boot and install and run from the eMac 2003 install CD, at http://macintoshgarden.org/apps/emac-g410-ati-restore-discs (download #9) - this might work with the MDD, but I haven't personally verified it.
2) the fan control works a little better on the older systems. Mac OS 9 support on the MDD was a little slapdash, Apple did it because they had to, but you can tell the MDD is explicitly intended to be an OS X computer. If I remember correctly, ultimately what happens is the driver loses track and the machine runs the fans at fast speeds all the time, which, because the MDD has several fans, can be a little loud.
3) Most (not all, but most) MDDs come with dual CPU configurations and dual CPUs are advantages to very few OS 9 programs. Any OS 9 program that *can* use dual CPUs will almost certainly be a better experience if you just used it in OS X. Especially given that the MDD can run 2GB.
4) Mac OS 9 can't reasonably use over a gig of RAM. It can probably boot with it installed but IME most workflows don't need more than 256. I was editing DV with 576 megs of RAM back in the day.
Mostly, in situations where you've got both kinds in front of you, my personal recommendation is to pick the QS if you're looking to focus on 9 and pick the MDD if you're looking to focus on X, but either will run either "fine".
I personally wouldn't buy any of these machines based on Zip drive availability - at absolute worst, you can/should use a USB or Firewire zip drive or run a drive without the bezel. On the MDD, you should be able to mount a 3.5" device in the second optical bay, and you can just fold down the flap when you're putting media in. Notably, I personally have no recollection of ever seeing a Zip bezel specific to the MDD, so I'm guessing this is what the MDD's Zip750 installation kit did, but I could be wrong, I looked, but not very hard.
The main thing that differentiates the QS'02 and makes it require newer OS install media than the dA/QS (which, again, are AFAIK literally identical) is that some components of the Ethernet interface changed and it requires newer drivers. The technical specs are otherwise basically the same and any dA can be upgraded with QS, QS'02 or third party parts to run at the faster speeds.
Here's the reasons why I'd choose a dA/QS/QS'02 for OS 9 over an MDD:
1) guaranteed ability to boot and install and run from the eMac 2003 install CD, at http://macintoshgarden.org/apps/emac-g410-ati-restore-discs (download #9) - this might work with the MDD, but I haven't personally verified it.
2) the fan control works a little better on the older systems. Mac OS 9 support on the MDD was a little slapdash, Apple did it because they had to, but you can tell the MDD is explicitly intended to be an OS X computer. If I remember correctly, ultimately what happens is the driver loses track and the machine runs the fans at fast speeds all the time, which, because the MDD has several fans, can be a little loud.
3) Most (not all, but most) MDDs come with dual CPU configurations and dual CPUs are advantages to very few OS 9 programs. Any OS 9 program that *can* use dual CPUs will almost certainly be a better experience if you just used it in OS X. Especially given that the MDD can run 2GB.
4) Mac OS 9 can't reasonably use over a gig of RAM. It can probably boot with it installed but IME most workflows don't need more than 256. I was editing DV with 576 megs of RAM back in the day.
Mostly, in situations where you've got both kinds in front of you, my personal recommendation is to pick the QS if you're looking to focus on 9 and pick the MDD if you're looking to focus on X, but either will run either "fine".
I personally wouldn't buy any of these machines based on Zip drive availability - at absolute worst, you can/should use a USB or Firewire zip drive or run a drive without the bezel. On the MDD, you should be able to mount a 3.5" device in the second optical bay, and you can just fold down the flap when you're putting media in. Notably, I personally have no recollection of ever seeing a Zip bezel specific to the MDD, so I'm guessing this is what the MDD's Zip750 installation kit did, but I could be wrong, I looked, but not very hard.