Content is considered to be the most important aspect while doing any of the above methods. Both are useful, you can start with Forum Posting in related niche & help other members with valuable responses. Actively participate in discussions going through, contribute your best towards, you can make use of backlinks in the Signature following the Forum guidelines.
Social Bookmarking is an effective approach towards page indexing, caching and more traffic. It also helps towards Keywords SERP (Search Engine Ranking Position).
Social bookmarking and Forum conversation both are good but when we talk comparative analysis than forum conversation is stronger than social bookmarking because in forum sites we become always active and get long term back link and traffic benefit.