For every forum that allows you to advertise, there will be five others that will help you on your way to getting banned. It's easier to get by with a blog, since you're not advertising a whole community, other than your own personal thoughts.
Building your community off of other established communities is a great idea, and very helpful if you don't have any other way of establishing that foundation. Since most forums are free and you don't have to spend much time to find people who are familiar with forums.
Promote your website in forums relevant content is essential because the backlink placed in areas with content related to your site. Google appreciate those backlinks.