Forum posts if they are do-follow and relevant can boost rankings really nice. I don't know about blog commenting, never bothered with that or know any high do-follow ones
As far as seo goes they are both bargain basement techniques, they could be argued away as community building.
As an aside, forum posting can bring a nice add to organic traffic, especially if you spend time and make an impact then people will click on your links. If you come across as spamming then not so much.
I'd avoid those as it's a waste of time building such links manually. They're however very useful when used as tier 2/3 links and built in volume using GSA.
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