I think both are very good optimization techniques in SEO. Both are having their own capabilities in SEO works.

It depends on your site....If you are doing blog posting the content must be unique and if social bookmarking the do in high pr sites it will help you.

Do blog comments in do follow websites apart from social bookmarking.

blog comments are better than bookmark but remember do follow links. No follow is waste of time. it can only increase traffic a bit if a person watches comment and click on link and other ranking values are not measured by no follow.

According to me, Blog Commenting is best.

Blog commenting have a great role to get the traffic of website.

Blog commenting and social bookmarking both are very important for backlink. Blog commenting is good for quality backlink but we get first backlink from social bookmarking.

Before i reckon blog commenting is better but now follow up to develop of social bookmarking. I think it is the same important that every part is needed for quality backlinks.

That is actually an effective strategy for your site is traffic. Some blogs will attach a no-follow attribute to your links, this means Google and other major search engines will not give link weight age to that link but you do get the benefit of traffic.

For me.. both blog commenting and social bookmarking are the best to get your site index by Google. However, I've seen plenty of bookmarking sites have been de-indexed and have no PR which is not really good for your site. Also, most blog comments now are nofollow but if you submit your site and give valuable points to the topic.. it can add or drive traffic back to your site..

Blog commenting is very helpful way in terms of driving traffic to web sites

social bookmarking is the best back link compared to blog comments.

do "No Follow" comments help in ranking and how can we find do follow blog comments ?

Both are good technique for driving a traffic but I'm still confused after the panda & penguin updation "Google says don't target keywords more"..If we are not target keyword the how can achieve a good ranking on google?? Can you guys explain me??;)

As per my view,
Social bookmarking is good.

go for social book markings compare with blog commenting as they are no follow links , while Social book markings are do follow and can get much traffic and easy to submit and approved.