I think we could use both of them. While, in my own experiences, forum signature links would have an extra benefit of that we could change the anchor text in batch mode according to our keyword strategy modification. :)
I also believe that if you use both then you can get more benefits rather than giving preference to any one. Because you can get more and more traffic from social bookmarking and forum posting is also best wayto increase traffic as well as to get more backlinks.
For me, Both are important. Especially when you are a SEO. As jannethden says, both are important you can't afford to leave any step.
But the real question is, Which is more beneficial right?
For me, I think Forum Posting can be much useful than Bookmarking. Because some bookmarking, can be deleted right away.