What kind of directories? Is the blog established? bad directory links can tend to look spammy, as far as SERPs, both. If I had to pick one or the other, blog posts
I would say blogposts are better. A directory typically links to thousands of websites, while a blog usually only links to hundreds of websites. Also a blog is full of unique content while directories are full of duplicate content site descriptions. Most directories are generally themed while a blog has the potential to be much more relevant to your website. With directories you often are forced to use your exact site name as the anchor text which can be another drawback. There are really only a handful of directories worth submitting to. The rest help very little and are probably not worth your time.
Depends on whether the links are relevant and the PR of the page where the link is and how many links, link out of that page.
I agree with vansterdam. Although what shows up in the backlinks lists provided by the search engines aren't necessarily the only ones the search engines see and weigh, it is interesting to note that even when some of my sites are listed in well-respected topical directories (like prominent PR6+ directories of lawyers and law firms), the links rarely show up in the lists of backlinks. On the other hand, if I comment on a blog, there is a very good chance that will show up in the backlinks.
Blogs will always be better if it is a review-type situation. Otherwise if it's just a comment, I'd go with a directory. Too many blog comments discredits the link you would post. Although directories are fairly useless too unless you get a front page link. Bottom line: A review-type blog post will always win. Or a front-page link on a directory.
IMO blog post is more important. Because now days directories become paid and also lost their PR. While if you post in your blog, and do social bookmarking for those post it will be very helpful to derive traffic as well as get boost in SERP. Try to post very very latest news about your products and services. So that visitors attract and subscribe to your blog.
if your comments are getting approved and link you place is do follow in blogs then blogs are more useful then directories ... otherwise you can do both...