I am currently evaluating Ubuntu and Arch Linux for our server. Which one is the easiest to manage? Which one is better in terms of security? Does having a large organisation like Canonical backing Ubuntu an advantage over Arch Linux?
Having a large organization is helpful because you have the option of buying commercial support. I personally wouldn't go with Arch simply because there isn't a stable branch. Rather, everything is cutting edge. I know you can chose not to update your Arch install, but I'd still rather go with a distribution where stability is a priority. That being said, Arch is surprisingly stable given its rolling release model.
You also might want to take a look at Debian, which has a track record of being stable. Honestly though, Debian is a bit stuck in the past, as seen with the delays in the past two releases. They really do care about stability though.
There are two sides to stability: "stability through lack of change" and "stability through tracking upstream". I seem to get bitten by the former more frequently than I'd like with certain distributions.
Then there are problems with upstream some times, this Mysql 5.1 GA release may end up being problematic for distributions that track upstream really closely. But I consider that to be more bleeding edge than not.
I'd like to see some distributions smartly packaging both a considered stable version and a closer to bleeding edge version, without goofy include-the-version-in-the-package-name cruft (except in the case where upstream does it, like sqlite did with sqlite and sqlite3 naming), so it's easier to pick and chose and move between versions without double installs and a lot of conflicts. Its more likely easier to just stay on an older distribution release than it would be to maintain this.
Sometimes I have to spend a lot of time in RHEL or Solaris hunting down packages of software that I would find and install in Arch in a minute.
I haven't had worse problems with Arch than with 'real production server operating systems' like the two above. I am developing my personal stuff on a VPS running Arch.
archlinux.org runs on Arch. We had some downtime earlier this month, but I think it wasn't software related.
Business guys, however, don't want something new and cool. They want a conract that allows them to yell at other people when things go wrong. That's why Arch might be a tough sell sometimes. Perhaps an idea for somebody's startup, mm? :)
I like Arch, but it seems not so geared toward server use as Ubuntu is. That is not to say that it could not be done properly, but Ubuntu Server would lend itself better out of the box.
In terms of security, they will be about equal under proper administration practices.
Canonical backing Ubuntu is probably an advantage for a production server environment.
You also might want to take a look at Debian, which has a track record of being stable. Honestly though, Debian is a bit stuck in the past, as seen with the delays in the past two releases. They really do care about stability though.