I think you'll find that your team will like either Trac or Redmine more than Bugzilla or Mantis. Both integrate nicely with Subversion. Both include wiki, forums, project management features...
Quick overview:
Trac: Very widely used and loved, written in python, huge community, lots of "plugins". A common complaint is that it doesn't support multiple projects out-of-the-box, but you can add a plugin to help with that.
Redmine: Written in RubyOnRails. Like Trac, but more complete out of the box. The authors of Redmine are trying to create a better Trac than Trac.
If you are interested in what others searching for bug trackers have written, comparing trackers to each other, I've put some links together here:
http://ifdefined.com/blog/post/2007/10/Links-to-other-comparisons-of-issue-trackers.aspx
If you on Windows, which I'm guessing you aren't, then also consider BugTracker.NET, an easy-to-use, very configurable bug tracking system in .NET/MS SQL Server. (Disclaimer: I'm the author).
