Most everyone will say FreeBSD. Most everyone will say so because they have been told it's "better". However, freebsd does rock as a hosting platform, so they can be forgiven.
We actually employ a somewhat customized version of Debian Linux. We have our own packages for some common apps like apache, php, etc. This means we can upgrade the package, put it up on our local package mirror, and all our servers will sync to the new app automatically nightly.
This is a lot like "ports" in freebsd, but we prefer it. Namely because this is what we have experience in, and can run it in our sleep.
So.. before the Linux/FreeBSD war starts. Let me preface by saying use what works, and what you're familiar with. Linux and FreeBSD have gotten to the point where they are virtually indistinguishable performance wise, and reliablility wise they are the same as well in qualified administrators hands.
THE number ONE factor in which will work better, is what you or the people who administer your server are most familiar with, and comfortable running.
So.. let the flames begin!
-Phil