Forums are intended to go that one step ahead of blogs. Its takes off where the blog stops. It allows the author to write his/ her thoughts and invites the other readers to pen down their thoughts in reply/ rebuttal etc. Forums are intended to act as a platform for participants to freely exchange their views, without fear of censure, its a platform which supports the freedom of speech. It is intended to be the place where appreciation meets criticism to find the fine balance of the two, to draw the optimum picture of the situation / topic of discussion.
With reference to the Debate Mansion, Blogs may not be as effective as a forum, provided, of course, the Forum permits a open discussion/ debate on topics.
Blogs permit censorship. Thoughts not to the liking of the author may be taken off, it is indeed the private space of that author and he is the king of all he surveys in that space.
Forums are intended to be a open platform to permit discussions. Forums differ from blogs in this very vital area. Forums, normally, do not permit excessive censorship, for under the censorship regime, freedom of speech may suffer.
I remember the words of an old friend, who once reproduced an age old definition of the Forum as a "open place for exchange of thoughts". Its like a market place, where many topics of discussion is up for grabs. Take the one of your liking, voice your thoughts, if you dont like the heat, you are welcome to get out of it. If you like it, you are welcome to stay. But like a market place, where some law and order rules like "no goondagardi" etc. are imposed, some rules may be imposed, to avoid conflicts. But rules which clamp down on a persons Freedom of speech are akin to "Emergency Raj". These are fine during war, but during peace, it is killing the economy. And no war should last forever.
Sometimes, Blogs may be better than forums, if it allows you to express your thoughts, in an open and fair manner.