I have an account with each, but the only reason I have Myspace is because I have some friends who don't use Facebook.
Some people like Myspace's customized layouts. In my opinion, they're its worst feature. Yes, even worse than the never ending stream of fake spam women constantly trying to get me to add them to my friends list.
Myspace makes my eyes bleed. Some of the layouts are mind-boggling. On Myspace, a layout's legibility is inversely proportionate to its popularity. Black font on a black background is not good design. Further, I absolutely hate the ridiculous animated .GIF's in most people's comment sections.
On the other hand, Facebook's obsession with "Applications" is really irritating, too. The vast majority are worthless, and require you to install them to participate. That being said, the layout is much cleaner, and so is the clientèle. Myspace feels kind of like the bad part of town. The kind of place where you might not be in imminent physical danger, but you'll at least have to put up with hookers and panhandlers at the gas station.
As far as security, Myspace seems lax, at best. I'm constantly getting posts from my friends pitching MLM and pyramid schemes. Or, more correctly, I'm getting posts from scammers/spammers who have hijacked my friends' login credentials for their purposes.
What do you guys think? Any preferences?