It primarily depends on what you're writing about.
First off, you should know the difference between comments and forum posts.
Comments are one-way communication mediums. People read your blog post, and comment it. You can usually answer them but you can't keep up a discussion in comments. The comment section will grow and grow, the comment section will blow, your site will look like crap (unless you have a nice button that hides comments on click). There are formatting issues, problems with char limitations, and so on. Comments are not suited to discuss, they are suited for short feedbacks, and little suggestions.
However, forum posts are suited for discussions. In forums, you have everything you need. You can write bold or italic, you can link, you can quote, and the most important thing: you can write as much as you want. In forums, you indeed are able to keep up a discussion. You have much more control about your two-way medium, and the direction it takes.
So, what do you use? Depends on your content.
If you write about something minor, e.g. your opinion, or facts that have been proved several times, you should use comments. There's no need to start a discussion on it because it's your opinion or proven facts.
If you write about something really important, e.g. things that must be changed, facts which can't be proved or disproved, or your critical opinion about something important, you should use forum posts. Your readers clearly want to discuss it because not everybody will share your view of point. Comments are not suitable here. If you write, for example, about atomic energy, you will definitely create a topic to discuss.
But one important thing: never use both. This has social and technical reasons. In the first place, you can't maintain the comment section and the forum page on the same site, so people must switch sites to gain the full insight about the discussion. That's awful and discouraging. Also, there will always be people who just use the comment section or your forum. If that happens, you have two different communities. One about feedback and suggestions, one about important discussions. This would be a lot of work since you have to moderate two communities on one site.
In the end, you should be really clear on what you're writing about. Then you can decide which option is more suited for you. It's recommend to just use one option.
- Primarily opinion, fact, (or satiric) based blog = comments
- Primarily important and serious topics, or critical views of situations = forum posts
- Primarily passive and 'lazy' users = comments
- Primarily active users who are keen to debate = forum posts
