I've been using Ubuntu for a couple months on my server, and its been perfect (though its not too much traffic). I mainly used Ubuntu because thats what I have on my desktop, so I was comfortable with setup/installation/config/etc. If you use linux on your desktop, I'd stick with that, just because a production server isn't the best place to experiment. If you're new to Linux, Ubuntu is great to start with because theres lots more documentation for it than for other distros.