I've never tried B2, so I cannot comment on it myself...but I am a big fan of movable type.
I've used the software for almost 2 years and really adore it. You can completely customize HTML and CSS files -- it's quite easy for anyone familiar with these. MT has a valuable support forum that has an active user community. In fact, just recently MT came out with an updated version to tackle the problem of "comment spam". I'd say that the MT creators are very in tune to the needs of users and are always on the lookout for the means to refine what they have.