I've done numerous jobs on both wordpress and movable type and admittedly it is very hard to come to a conclusion about which one I prefer. I use wordpress as a content management tool for slickricky.com, but I use movable type for at least three major clients. If you forced me to choose between the two today, I think I would give the edge to wordpress.
Movable Type did just come out with a major upgrade, and it is vastly improved. However, I think (besides a few features) it is playing catch up to WordPress. Many of the navigation features are taken directly from WordPress, which shows MT's inspiration for some improvements. The major gripe I have with MT's upgrade is that it is extremely slow on the back end, if you use Internet Explorer. WordPress is very light, and coded to move quickly. For some reason Movable Type decided it was better to be graphically pleasing rather than efficient. In my opinion not a great idea. I am coming from a different perspective than many bloggers, because I have to code and reload pages hundreds of times with these tools, but either way, a slow down is a slow down.
The one major edge I would give to Movable Type is (when you have a commercial license anyway) the support is very good, and you can email a question and have an answer within a day or two. If you're not used to working with blogging software that can be a huge advantage.
When it comes to plugins, I would give the advantage to WordPress. Both systems have great and numerous plugins, but from the coder point of view. WordPress plugins are easier to write, and therefore you've got to assume that they will be a better quality over time.
What do you think? I'm sure I've missed some very important points, and this is quite an important issue that deserves some discussion.