In the beginning, it seems to make more sense to build a blog and drive traffic to it through forums. After you get good at the blogging and have developed trust from your readers, building a forum would be considerably easier. As you use both, you understand better the needs of both media, but blogs are much easier to build, maintain, and control on your own. Forums might take some help from people you can trust.
Blogs are easier to market and monetise. Trying to populate a forum with active participating members is very difficult as people only want to be in high traffic forums.
If you have a direct sell/membership Forum then I think you can do well. Otherwise I'd stick to blogs. I finally took down my last Forums about a year ago. Just too much hassle dealing with all the Spam. Even with filters and anti spam measures in place. Much easier to deal with Blog posts and comments than moderating a group of users who can post at will once signed up.
Blog = small village Forum = Big city ( if active ) Blog: You have to update the content Forum = Your members will do it for you, free Both of them will ping search engine automatically. Hope this help.