A blog usually belongs to a single person. Or in some cases, a small group of people. The main body of content on the blog is the responsibility of that person. That might mean that they write the content themselves, or that they act as a kind of editor/reviewer presenting someone else’s content. A blog also consists of relatively substantial individual entries, which may or may not prompt a discussion (the blog owner has editorial control of the discussion). The discussion might then involve a wider range of people. The quality of a blog largely depends upon the main blog entries, although that quality might then be enhanced by the subsequent discussion. Some successful blogs contain no discursive element. For most blogs, the role of discursive content varies over time.

A forum is very different. It is owned by all of its participants (although there may be editorial control imposed by a moderator). Forums are built out of shorter discursive exchanges. They are driven by a group of individuals, each sharing a strong need for such exchanges. Without that need, the forum will fail. A further drawback is that a dialog built out of lots of small exchanges is likely to fragment. Some forums avoid this because their participants have a good shared understanding, and are already driven to collaborate effectively. Other forums work because they are driven by an effective moderator-tutor.

Which would I recommend? Unless you already have the kind of community that I describe as necessary for a forum, you should probably go with a blog. To make the blog work, you will need to regularly add entries that are substantial and interesting enough to provoke a worthy discussion. One further possibility would be to invite selected members of your target audience to write entries for the blog, so that it works a little like an edited journal.