Both Content and back links are useful for ranking and Promotion of a website, both help in increasing popularity, ranking and traffic of a site. "Content" which is most important in improving SERP. It should be unique and fresh. You must focus on quality content, then your keywords ranking will automatically start increasing,
on the other side "Link building also works" like blog commenting, directory listing, guest posting, bookmarking, classifieds. But if you want to increase ranking, traffic or links you can hire expert from sites like SEOclerks,