Never heard of XBMC. Mythbuntu will generally have the same pros and cons that Ubuntu has. On the plus side, it should generally be easier to set up and maintain than most other distros, especially if you're coming from a Windows background; and there's a large user base (so you're more likely to be able to find help online). On the minus side, Ubuntu (and by extension Mythbuntu) tends to be pretty bleeding edge, with half-baked (or even quarter-baked) features sometimes making it into official releases. If you're not planning to move on this immediately, you might want to consider waiting for 10.04 (due in April); this is one of Ubuntu's "long term support" releases (which they do every couple of years). LTS releases focus more on stability than bleeding edge features.
The years just pass like trains. I wave, but they don't slow down.
-- Steven Wilson