Well, in my opinion the forum posting is much more effective than the blog commenting. Since, Google values links from new and fresh content pages the forums are updated frequently, thus provides more weight.
Forum posting is most effective way of getting backlinks. Another specific aspect to be kept in mind is that building links are more of quality than quantity. You have to take care to generate more links form useful forums than posting at all the forums at will. You can plenty of backlinks with simple forum building.
Blog commenting is much more effective than Forum postings, as the blog owner reserves the rights to accept or reject a post> it is little more cumbersome but more effective. you will see a lot of posts here in this forum only that offer nothing great to the forum, but are here just to increase the backlinks or getting a hit by mistake..
In my opinion Blogs commenting is much better as you can get real and targeted traffic in them
Hello dear friends, Robert S. new here, my reply is here, "both are useful but blog comments more useful for increase the blog page rank and forum posting (on blogger forum) helps to increase traffic."
Both are important and both are necessary, both are seo factors and need to be regularly but one thing i would like to mention that you will easily get a do follow forum but not a blog that is the difference.
It doesn’t seem to make any difference whether or not you have a dofollow plugin installed as far as the amount of spam you get. Some spam is directed at the blog owner, rather than readers because the spammer know that their comments would never get published.