Both Ubuntu and Fedora started out as Desktop-centric linux distros. Since then, Ubuntu has started releasing server distributions as well as its "LTS" (Long Term Support) server version. At this point, I don't believe Fedora has a server version, though I could be wrong.
I'd recommend you stay away from Fedora for server purposes. The upgrade cycle is just too quick. The Ubuntu LTS version is a decent candidate, though.
Ideally, I try and stick with either CentOS or Debian for servers. You ought to give Debian a try - Ubuntu was originally based off of Debian and nearly all of the tools (package management, service control, etc) are identical between Ubuntu and Debian, so you'll feel very at home. I feel that the Debian package maintainers do a little better job testing/fixing packages before they get promoted to the "stable" branch. As such, there's less of a chance that an upgrade will break something.
With regards to the ease of installing Bugzilla and Subversion...subversion is easy, of course:
$ apt-get install subversion. If you're going to use svn+ssh, that's really all you'll need. If you'll be using apache and DAV, then there are a few more bits to configure, all of which are well-documented in various places on the web. With regards to Bugzilla, it really depends on how recent a version you need to run. The current debian stable package for bugzilla is v3.0.4. Contrast that with the most recent tarball releases, which are in the 3.6 release candidate phase.
Speaking personally, I prefer to use the distro's package management for what I call "base" services - things like apache, mysql, userland tools, libraries, compilers, etc. For applications, I usually install from the official tarball. I find that the packaged versions of applications sometimes put things in odd or unexpected places and as such, you sometimes run up against resistance when trying to get help in that application's support forums, mailing lists, etc. Also, I find that upgrades are generally much easier when applications are installed from tarball, as you can just follow that application's upgrade guide. All of that to say that I'd recommend that you consider installing bugzilla from it's official stable tarball.