If you've never administered a system before, the transition from being a regular user will be difficult, regardless of which OS you choose to learn to administer. The difficulty stems from the wider shift in thinking that's required. Instead of just making sure your room is clean, now you have to run and protect the whole apartment building. But the difficulty is also educational and rewarding. (We realize they also told you this for your theoretical physics class in college, but we are not lying.) Learning to maintain Ubuntu servers is a great choice for you because you'll benefit from a vibrant and helpful user community, and you'll be working with a top-notch OS every step of the way.
If you're a seasoned administrator who came to see what all the Ubuntu Server fuss is about, stay tuned. The project, though rock solid as far as stability goes, is still in its feature infancy, and the Server Team is working very hard at making it the best server platform out there. We're emphasizing advanced features and we're being very fussy about getting all the little details just right.
In both cases, if you're installing a new server, give Ubuntu Server a try. It's a state-of-the-art system, and we're sure you'll enjoy using it. Get in touch, tell us what to do to make it better, and lend a hand. Help us make Ubuntu rock even harder on big iron and heavy metal!