Well I think Linux generally is very reliable but I understand you're concern over something so new. A good point for Ubuntu is that they'll be continuing to support and upgrade this version for 5 years meaning you wouldn't have to reinstall in a years time to catch up with software updates.
But really I have no experience with Linux when it comes to servers, only FreeBSD and a small bit of OpenBSD. Both I loved and would be using one of them if I were setting up a server again, however if I wanted a GUI on my server I think I'd opt for Ubuntu, hence the reason why I recommend it. Also I've used it as a desktop and really linked it, and it proved stable for the short period I stuck with it before moving onto my current beloved;
Zenwalk .
In truth almost any Linux distro can be setup as a server so you should be choosing one you're comfortable with, I'd recommend choosing one with live web updates, which an awful lot have but is still something to double check.