I've done blog commenting and done guest posting. Guest posting works like at least 10 times more compared to blog commenting. So i doubt anyone should waste time on blog commenting. Blog commenting mostly is a waste of time anyways, since you can leave your comment on a page with some good PR, but the actual PR can be (and mostly is) 0. So basically you don't get much from such a backlink. When you get a guest post, you get your backlink on a PR0, but later that page gets bigger PR, you just can't see it till the next update, but it is out there all the time and since your link was added before all the link juice was floating to that page, your backlink gets a lot of benefits from that page.
Good example: Some time ago i had ~20 comments on pages with PR3-5 (actual pages, not root domains). After recent google update, i've checked these pages and most of them are PR0 now. My rankings haven't increased at all.
I published guest posts on ~15 blogs (before update), after update i've checked these pages where my articles with links are, and i can see that only a few has PR1, one PR2, the rest PR0. My rankings increased quite decently with all these guest posts.
So just once again, blog commenting is a waste of time. Even if it is good at some point, but the value is still low + all the benefits are very temporary. A lot of times your comment can be removed later or that page spammed to death by other people with their links. And like that you actually can get negative results by having a link on a page with shitload of other spammy links.