It depends on what you want in a social networking site.
If you want to meet new people, Myspace is for you.
If its purely for your current friends, go with Facebook.
For an article for my school paper, I did a lot of research into the safety features of both sites, which is the main concern with most people because the sites serve basically the same purpose. With Facebook, members must be invited and confirmed by a network, so that means no random old skeezes. Its not 100% safe, but they've been working on a faster, more effective turn around for abuse complaints. Plus, you can choose how much information individuals see. Facebook is getting more and more customizable. Over the passed summer, they've added a lot of new apps and cool stuff, so if you have a lot of people already using it, I highly recommend Facebook.
I think Myspace gets a bad rap, but if you're going to keep using a myspace account, please please PLEASE be careful. A lot of the myspace related incidents and tragedies have to do with people not being careful about who they talk to and who can see their information. A myspace profile is much more open to the public than a facebook. But, you can customize it more, which is pretty fun. Also, myspace has a much larger music and video community. Their band sites are amazing, but you don't need a myspace to look at them, so you can really go either way.
Facebook WAS created for a more mature audience. It was started by Harvard students a couple years back and has expanded over time to other colleges and highschools.
I guess if you're going to pick, just be smart about using either of them. Don't be the person who has their age, street adress and full name on their page.
Happy Social Networking Hunting!