I can't answer this all as I am not heavily involved in this aspect but I can tell you that the Gallio project is the new iteration of MbUnit (perhaps my wording is off here but Gallio is basically picking up where MbUnit left off and just utilizing it to avoid writing all that logic again is my guess). Again, I wasn't involved in this (or the mbunit/gallio projects), but I believe the reason we need Gallio vs nUnit is we needed something that is more specifically ASP.NET friendly, not just.NET friendly. 

One of the "cool" features of MbUnit is that you can turn on screen captures and videos of the tests as they occur.  This is fantastic for being able to watch how a test fails thus making it much easier to repro a bug and shortening the turn around on bug fixing in regression tests.  Check out the solution we provide and you'll see you can turn on video in the app.config file to get a video embeded in your test results report.

MbUnit also made it very easy to provide data to tests so they can be run multiple times with different values, check out the way we did it with a csv file so you can manage the data in one place.  Very neat.

As for documentation the way we wrote the tests was with community members in mind who wanted to learn how to test.  Jeroen from Watin has also approved the way the tests are written so we are confident this approach works well.  If you write some tests for the core or "core" modules please email me and we'll review and add them to the project.  We are always looking for help to get a larger suite of tests sorted out.

HTH your decision.