In that case, if you've got the time to spare the best course of action has to be that you download the operating systems in question and play with them for a while. Find out which appeals most to you. Set up an apache/postgres environment and keep it running for a little, learn how the distribution handles upgrades for you (or if it doesn't). If you're trying to find out how the OS handles load, run a stress test (or post a story to slashdot ;-). I'd definitely recommend looking into Debian, it does not have the glitziness of other Linux distributions but is very solid and a joy to keep running.

Another thing to do would be to hang around some of the mailing lists/forums associated with the distributions you're looking into. You're bound to run into issues where you need help at some point, and the different communities work in different ways, so it's helpful if you feel comfortable in a specific community. For example, another OS I could recommend from a security aspect is OpenBSD, but newcomers often find their forums extremely harsh, while others enjoy the high technical clue to bullshit ratio.

"The best choice of OS" depends on many factors and will often change from project to project, so its worthwhile to at least have rudimentary knowledge about how things are done in other environments.

And welcome to the world of *nix.

Update: I forgot to say, this may seem like a bit much for a *nix beginner, but take a look at Xen, this allows you to run several virtual OS images on one machine at the same time. The guest OS kernel has to be modified in order to run under Xen, so at the moment it only supports Linux, NetBSD, FreeBSD 5.x and ReactOS, but I've heard rumblings that Sun are working on getting Xen into Solaris as well, and once the new virtualizable CPU's from AMD and Intel are out (sometime next year) you'll be able to run any OS as guest. Useful for testing as well as production.