Neither blog nor forums is better over others and the only benefit each method get is the format of content.
Blog has certain advantages over forum not because they are blog or forums but the way they function.
1. Blog tends to get more fresh content over static page site and so people having a static page site and blog tend to assume blog ranks well.
2. Forum tend to get good content but the on page factors are never great like Title of the posts and Hx of the posts.
3. Forum may have the posts which are actually useful low down the page.
The following factors make the SEO experts believe blog is better but then most important thing is always the fresh and relevant content.
I prefer blog comments. You can go with blog commenting to get Quality back link. Don't spam on blog while doing comment posting on it. Do prefer high PR blogs for submission. Also wordpress and blogger is the best one. Good luck.
According to seo point of view blog and forum both play an important role in optimization campaign. Blog is a personal website whereas forum is a community in which you can share your thoughts.
I have always read in forums that Blogs seem to do well than other sites and specially forum.
Both blogs and forums are of equal importance.
By blogging we can improve the number of visitors.
By participating in forum with signature will improve web site traffic and also to find more customers for your website