It really depends on so many factors... domain age, theme, linking page and total outbound links on that particular page... So you cant really state that one is important than the other
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If you do blog comments, it is also a powerful but make sure that any blog you use has a minimum of 6 months or more of archived blog posts and that it has a valid Google cache and at least some kind of www.technorati.com profile. Blogs in the top 500,000 with an authority score of 10+ are just going to be more effective, hands-down.
Its is advisable to use blog commments, to generate high page rank or targetted traffic, while directory submission, would act as a source to index our website more often and getting higher rankings.
If you want the quick lot of visitors to your site then you should surely go for the blog comment else you should go for the directory submission. As directory submission will surely help but on the long terms. It depends on you which type of traffic and in which time you want.