What distro should I use on my file server?

A friend of mine just asked me what distribution to use for a file server in his lab at another university. His local tech guy had been pushing for Ubuntu. He knows his way around Linux, but almost entirely as a desktop end-user. Here is what I suggested:

Ubuntu is definitely one of the more solid distributions out there. I'm glad that we're getting our desktops away from older FC distros and on to Ubuntu. One nice thing is that stanford maintains a local ubuntu repository so updates are always fast.

Ubuntu is designed around being a desktop distribution, but they also created a server distribution with a similar philosophy. I'm not sure that it translates all that well. One thing it does do nicely is not try to install a lot by default, but if I were going to build a server I would I go with Debian or FreeBSD. I feel like Ubuntu made some compromises in terms of security and structure to make linux more accessible for desktop users, but in my servers I'm perfectly happy to have my system inaccessibe as long as I know my way around. However, if you are going to be one of the primary maintainers, Ubuntu might be great for a server. In a lab environment behind firewalls security isn't as big a deal as for public servers.

When setting up a file server there is ZFS on the horizon. It is has been getting a lot of what I believe is well-deserved hype. It is easy to administer, fast, and stable. However, it was invented by Sun and is under the CDDL license which is incompatible with GNU and Linux. There are currently projects underway to port ZFS to FreeBSD (scheduled for release 7.0) and into the FUSE framework to allow it work in Linux, but at the moment the only way to get it in a stable distribution is to use Solaris or OpenSolaris. OS.X 10.5 will also have ZFS in it! It might be worth looking into a ZFS solution for your large data storage, but if you need the systems now I would recommend Debian or Ubuntu.

A note on Ubuntu versions:
There are lots of different flavors of Ubuntu, such as Kubuntu and Xubuntu and Edubuntu. They all use the same package tree and just have different desktops and default packages. Ubuntu can run KDE, Kubuntu can run XFCE, etc. Ubuntu is often considered the vanilla flavor. There really isn't much of a difference. For a server you'll want the Server version, and make sure to grab the LTS version. The Long-Term Support version is guaranteed to have updates available for the next 3 years if it's desktop or 5 years if it's server.