Actually, MT is a good piece software. IMHO it is the most advanced blog system available, because it's one of the oldest systems around and commercially supported.
It's advantages are Static Pregeneration, various DB/File storage methods, easy Multi-Blog administration and many available templates.
It's disadvantages are the licensing scheme, proprietary template code and for many people, it being written in Perl is a pain in the back. Also the static pregeneration can take up a lot of time when people comment, and it largely influences the dynamic possibilities. Of course you can use dynamic generation for the MT blogs, but that IMHO tends to be a bit slow.
The advantages of Serendipity over MT are:
* Licensing
* Full PHP integration, that means: Easier to code than perl.
* Common Template Syntax (Smarty), easy to adopt and use
* Spartacus Plugin System to add plugins on the fly
* Very easy upgrading to new versions
* Flexible WYSIWYG editing with many available editors (if you care...)
* Extensive Multi-Langual abilities
* Built-in media database
* Many german people using it, responsive developer community *g*
In the end, it's a personal choice. You should testdrive each system and try how well you get along with the user interface. Personally I find the MT interface a bit bloated and not so intuitive. But other people see that completely different.
If you tell you what exactly you're planning to do, I can give you a better overview if and how Serendipity is suitable for the task at hand. Try to get the same out of an MT developer...
Best regards,
Garvin