After several months of testing, I came to the conclusion that all of the tested distributions have an area where they fit best.
For the use as a desktop system, this is my opinion, Ubuntu (based on Debian). Ubuntu is simple, fast and has a clean desktop. Even inexperienced Linux users should be able to work with this distribution without much learning effort. From the administration point of view, the distribution has an excellent package management interface called synaptic. If you prefer the command line, you can still use the command apt-get as known from the Debian distribution.
For the use of a server system, I currently use FreeBSD. FreeBSD is stable, good documented and has a properly working security management. One big advantage is the long release cycle, which prevents half-year upgrades of the whole system. The ports collection helps me to keep the server up to date and tools like portaudit reports insecure applications (critical). All in all a good choice for a server.
Fedora Core is a good technology preview and gives me the opportunity to test things I can’t do with the stable versions. Unfortunately, the release cycle is to short for productive use but to get an impression of what is possible and whatnot, it’s a well-designed distribution.