Definitely blog. If written well and focused on topics that are interesting and relative to your keywords they can be a huge marketing tool. I am currently working on a site that has just 7 backlinks and the only marketing we are focusing on is blog, we have managed to obtain p1 rank under around 15 medium competition search terms in the last few months. However I would stress that the content has to be good quality, original and informative for it to have any ongoing weight for SEO.
