I think Myspace is still the better for music...so I guess it depends on your interests. Facebook is the bigger of the two and still growing so if you're interested in social networking then I would definitly sign up, and compare the differences with Myspace. It can't hurt to have both either - most people do, even if their Myspace is aged and not updated anymore!
I absolutely go for Facebook. Although there are features that I don't really like in facebook but overall, it's really a nice social network to get connected with you friends and family. Facebook has grown a lot and it has has so much improvements that even the users are a bit unprepared for its frequent changes. Sometimes to changes are undesirable but most often their improvements are really good. That having said, facebook rocks!
Facebook, of course. My opinion is quite racial, since I never really have MySpace. And I think almost all my friends don't have Myspace too. I tried visiting it once, and it FROZE. The page had an error, tried refreshing it, took about 2 minutes, and opened.. And yeah,my FI on the site hada THUMBS DOWN.