I am already biased towards bugzilla for 2 reasons - its proven in open source projects and its written in perl, while mantis is new to me and written in PHP.
After having to uninstall and reinstall various php binaries to get even a login screen I wasn't very impressed with mantis - the documentation is unclear and lacking.
The PHP code isn't very clear and I was glad when after wasting an afternoon and a couple of hours of this morning trying to get beyond a login screen I was able to give bugzilla a try.
Bugzilla installed easily enough, even before I read all of the documentation. The documentation is very helpful and the CPAN Bundle makes it a doddle to install.
We dumped the default redhat sendmail install and switched to exim.
Once I'd done that it *just worked* - no excuses.
Imagine my surprise, after being told by many people that the codebase to bugzilla was best described as a car crash, that it has moved to template toolkit.
With little effort and without even touching a cgi script I had branded and beautified the headers, footers and a couple of pages, adding my own toolbars based on those provided.
Once I dug into the database and read the documentation I was able to start hacking away, adding extra drop downs to forms to save typing users names, and even creating a new cgi script in about 10 minutes that gives a quick summary of current bug status and assignment.
There are many areas where the code could be refactored to fit in with the London.pm programming style : using an sql phrasebook and a nicer database abstraction layer as well as more use of templates for things like forms and result sets.
I could happliy hack away on bugzilla all day linking it into our systems for cvs, project management, documentation, etc.