While Blogs and Forums seem to work in similar ways - they both allow comments for instance - they are in fact completely different beasts.

A Blog is a sort of public diary - it's short for Web Log. A Blog is more of a commentary than a conversation (though comments are possible) I tend to look at Blogs as my personal news page, much the same as the front page of St John's online. 

Forums on the other hand are much more discussion focused,  - somewhere to chat with people about all sorts of things. If your intention is to start a discussion, then forums are a much better place to post. The structure of the forum is much better suited to discussion based topics.