Variation and relevance is the key. As long as your blog/forurm post is relevant it will be effective. Me personally, I prefer blogs only because there's less restrictions and my post will not be tagged as spam. Also don't forget to variate your anchor text.
i think the talk is about promotion of his website(s) on forums and blogs! if so you have to look for quality backlinks and the websites which are relevant to your content... forum posting bring traffic and blogs posting also if you mean with blogs (Articles) publishing...
Why are you not considering "Forums and blogs" instead of "Forums or blogs"? It will be beneficial for you if you use both techniques..Forums will help you to bring more traffic and niche blog commenting will help you to create a back link...