Movable Type is a great blogging system, but can get encumbered by licensing fees, depending on how you use it. It is nice in that it scales up pretty well--a great many of the web pages are statically generated, so that heavy web traffic will not bog down a CGI process or database server. The flip side is that it can sometimes take a while to post if many files need to be rebuilt, not to mention the possibility of something getting out of sync if the build breaks or gets confused.
WordPress is the Open Source blogging system that gained popularity when MovableType changed their licensing. It is quite powerful, offers easy plugins and themes, and is completely free. While the themes are a nice bit of icing, I can't say that I have seen more than one or two plugins that really wowed me. The main difference between WordPress and Movable Type is that it uses 100% dynamic pages, so there is a slight CGI and database hit for each page request, but I never found this to be an issue (even with the crazy traffic of the ilovebees moderated blog.)
Anyway, that's my $0.02. I personally tend to like WordPress over Movable Type, but that is mostly because I am much more familiar with PHP than Perl. "Your mileage may vary," as they say.
