With Movable Type, one danger is that you can quickly run into quite a bit of complexity with multiple blogs all running their own templates. Yes, you can fix that by restricting the blogs to a few central template files / style-sheets, but then you lose the custimizability that is one of the best parts of MT. The upside I see in MT is the straightforward way you can configure each blog with it's own URL and then lock down users so they can't mess with the internals / templates / etc... Plug-in wise, you'd probably want to consider at least mt-blacklist for spam and the permalinks hack to work around the way MT numbers articles in numerical sequence without regard to which blog is publishing it.
Running out of Xoops, I think you'd have to use both the WeBLog module and the one that lets you setup run multiple virtual hosts on a single installation. No experience with either of those tools, but I'm sure it could be done although I wonder how the backend.php RSS generator would handle it. The advantages I see with using XOOPS are the simplicity of offering blog owners & viewers) different themes using the built-in theme block and the ability to add many non-blog features.
Something you might want to investigate before making your decision is SixApart's product development plans. I recall reading somewhere that the two next big products after TypePad were 1) MT Pro, adding in the photo blogs & simplifying templates / styles & 2) Hosted TypePad for resellers. The later would seem to be ideal for your situation, maybe you can convince them to let you beta?