Very simply, a forum is an older version of self expression and is similar to that of a bulletin board. It is a niche specific thread, as it allows the author to attract like-minded people to their interests. The commentator is able to choose which thread they wish to leave their opinions on.
A blog is similar to creating your own webpage and typically has templates to choose from, meaning your page is automatically set up and just needs to be customized to your liking! It is similar to an online diary, where anyone can read and leave a comment.
A forum is more discussion based and tends to turn into something like a debate, which is perfect to stimulate conversations in the classroom. Our class tends to flow smoothly with using the forum, however a blog can do the same and tends to be more for ones who seek to express themselves freely across the internet.
Now what position do you wish to take? Choose wisely!