See
https://help.ubuntu.com/community/ServerFaq - the only real differences are a text-mode installer and a different selection of packages
on the CD (which is irrelevant these days if you have a decent network connection - they share the same online repository). It's one-line command to install the desktop environment - if you're worried about bloat, the server is the place to start, since it doesn't install any huge server packages by default and the desktop stuff is an order of magnitude more bloaty than the server packages anyway. What isn't clear is whether you'll get hardware accelerated graphics. Currently, though, if you're running under a vm,
Parallels supply the driver - and since Apple have shown Debian running in Parallels on Apple Silicon it looks like that is in hand.
However, Apple have said that they're not going to support direct booting other OSs and what with T2 and all that, that probably means it won't happen. Frankly, with good virtualization support, I'm not sure that's an issue.
I think the iMac Pro is a dead duck, Apple Silicon or not. From the timing of its launch I suspect that the iMac Pro was
going to be the new Mac Pro before the
mea culpa press conference when the Mac Pro was promised - which might even have been the result of sharing the iMP plans with a few key customers. Otherwise, the timing doesn't make sense: it's harder to squeeze a hot, sweaty Xeon into a slim all-in-one than it is to design a bog standard Xeon tower (and, sorry folks, the Mac Pro
is a bog-standard Xeon tower with a few added gimmicks).
With the shift to Apple Silicon, the existing Core-i/Xeon divide will change. What there
may be is an "Apple Silicon" (coming this year in the 13" MB Air/Pro and 21" iMac replacements) and an "Apple Silicon Pro" (ignore my made-up names) which I'd speculate would be for the 16" MBP and 5k iMac - which will either have a souped-up integrated GPU or enough PCIe to support discrete GPUs and other "workstation" features.
Thinking about the Mac Pro, since it only launched (effectively) this year and was a new line rather than an update, I think it will have to hang around (in its Intel form) for another 4 years or so if Apple want to keep any credibility. And I don't just mean "supported" - if you're a Real Professional using it in a Real Business then you'll need to be able to replace broken machines and buy new machines when you take on new employees etc. until you're ready to change workflow
again.
That
could be an opportunity for Apple to have another go at weaning pro users off the "Big Box o' slots" format: the trashcan failed for 3 main reasons:
1. People need PCIe slots and extendable internal storage
2. Apple had let the "classic" Mac Pro get horribly out of date (and discontinued in Europe) so people were forced to change
3. No suitable upgrades (that would fit in the trashcan's thermal design) came forth from Intel and AMD.
Now, the Intel Mac Pro sorts out (2) for the next several years and, with Apple rolling their own CPUs and GPUs (3) is also fixed. If Apple really aren't supporting discrete GPUs any more,
half the reason for (1) is gone - although that leaves people who need PCIe for audio/video/networking/storage etc. Now, to be clear, IMHO Apple should stop messing around and
make a straightforward tower system but I'm speculating on what they might do given past behaviour - also, if keeping the Mac Pro going kicks the can a few years down the road, things may have changed on the PCIe front by then.