Well you really can't compare them. Livejournal is for blogging, Myspace is for networking. Livejournal does have communities and myspace does have groups. But it just all depends on what kind of climate you want to be in. Myspace is completely different from livejournal. I have an account at both, but I only use my LJ to post at communities, and use my myspace to check out group and keep in touch with friends

With myspace though I forgot to add, you're kind of forced to see "pretty people" or young kids who lie about their ages and post rather... well, not so age appropriate pictures of themselves whereas at livejournal its less about image and more about writing.