Well, that is no fun, you just took the tinkering away
It is settled, Ubuntu at work and FC5 at play.
Nah, if you want to use Ubuntu-MythTV you can... but you might end up banging your head against a wall at times in frustration. (I know I did!) And as they always say, "your mileage may vary" in your experience.
Over the past year, I have tried MythTV on FC4, FC5, Knoppmyth, Ubuntu 5.10, Gentoo (ouch!) and Ubuntu 6.06.1 and a few others that I have tried to erase from my memory due to the pain involved. And FC5 seems to work the best and has a very active and helpful community. So I thought I would pass on some advice on my experience thus far. I investigated pretty well every major Linux distribution before settling on FC5 for a base.
Of course, if you are tinkerer, you are welcome to try and distribution you want... that is part of the fun.
BTW, I used VMWARE Workstation heavily when testing different distributions/configurations to make the process less painful. (Nothing like running 3-4 Linux distributions at the same time trying different things.)
Once you get the hang of FC5-MythTV using Jarod's guide, it is pretty smooth sailing and very quick to setup. I think a lot of folks get intimidated by the lengthy howto, but it falls onto the wordy side to cover as many possibilities as possible rather than to be brief.
And the bonus is you get a system you can tinker with to boot as FC5 is a very, very large and complete distibution. Tinkering with MythTV is practically a hobby for me. (I'm about to dive into how to directly alter the MySQL database to get what I want to happen exactly... yes, I apparently like pain trying to optimize everything.)
I must say your MCE setup seems mighty customized (what you are doing is not the domain of typical users by any means)... but now that it works for you, it sounds like quite a nice setup. Downloading HD via firewire is mighty cool.