While I agree with the previous posts that commenting is less effective for both SEO and driving traffic, I would like to add a couple of things.
1. Commenting should be done on sites that are related in some way to your niche. Don't comment on any blog just because they let you comment.
Your comments should also be relevant to the topic. I have seen many comments where it is obvious that the person who is commenting has never read the article.
Some bloggers do not read the comments, and will get comments that have been posted by someone getting paid 2 cents a comment. the end result is the comments are generic statements such as: "Great article - very informative". What does that say or mean? It means it's a blind comment - the article was never read.
It is important to comment on higher ranking sites that will give your comment a better chance to be seen and read. Some sites will let you comment, but will not allow a link. You want the backlink!
Finally, the only way that anyone will click on that link to your site is if your comment is extremely good, and adds to the article. If you don't know enough about the topic to create a comment that blows people away, then find another article to comment on.
2. Guest posting is a different animal. ALWAYS read the submission guidelines. Similar to doing commenting, you want to guest post with sites that rank as high as possible.
No matter where you post your article, it should be well-written, and should follow SEO guidelines. Many sites will tell you what they want, but some don't. Alt tags for graphics, keywords, etc. are important to keep in mind when writing the article.
If you can't write very well, and still want to guest post, find someone to edit your material. Quality and good information is the main thing that sites want for submitted articles. Why not put out great content? It is your name on the article after all, no matter where it is posted.
Remember the "1st Impression" theory? Your article may be the first time that the reader has heard of you. If your article really helps them, then they are very likely to click to your site to check you out.
If you do submit a post, and it is accepted, you are still not done. ALWAYS check back to your post and check your comments. Some sites will alert you when you have a comment, but not all do, so you must keep on top of the comments that come in.
Whether you comment an an article, or guest post, each is time consuming. You will spend a lot of time either way looking for appropriate sites to work with, and then you need to read articles to see what has already been written, and what the main focus of most of the articles are about.
It is a slow process, but many people get good results. It is only one way to drive traffic, and I personally don't recommend it being your only way.
TIP: Focus on only a few sites to post for, and a handful more for commenting. Once you get known in those circles, people will remember your name, and your expertise. This will often produce traffic, and targeted traffic at that.
