A blog is just a glorified informational site imo. SEs love it because by definition, a blog is primarily made of content and is updated frequently. They may get more traffic because of blog networks.

I dont think either one is better – some situations a traditional sites work, others a blog works. Many SEO folks suggest running a blog on all your sites to add content to it thats related to your subject. This works especially well on sites that sell something since there tends to be little original content on the sites themselves.