There are many discussions on the web about which one is better, and why one is more popular with a certain group of users than the other. Here is what I think why WordPress is more popular among the causal blogger than MT.
Note #1: I think MovableType is, in general, a better blogging system (e.g. more scalable, more flexible architecture, etc.) but oftentimes technical merits do not count for much when causal users evaluate software.
Note #2: I am comfortable in unix shell so that's not where I burnt my time on.
Fact: I've spent around 4 hours trying and failed to install the infrastructure (CGI and Perl's DBI and DBI::mysql or DBI::sqlite) on OS X to get MovableType to run.
Fact: I only need to spend around 30 minutes to get WordPress to run.
Lost in Colonisation 
I'll go with WordPress if it's main stream blogging if you mean. MovableType is quite famous for the executive types and those who really have the deal to deal with the tech especially in troubleshooting perl etc.
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