I've only played with bugzilla a little bit, but my sense was it was much more organized toward software projects developed by a team.

One of the things I like about ProjectPier (or Basecamp) is it allows differing levels of access, so for example, a contractor sees different information than the employees do, and there is a combination of tools such as to-do lists, milestones, notes and comments (some of which is similar to bugzilla.)

I will say that I suspect bugzilla is probably more mature and feature-rich than project pier, but both are free, so perhaps take a look at both of them. They do similar things but they were developed with different ideas in mind, so the way they approach things is a little different.