SEO-wise, both are significant. You need both to rank on SERPs and although it is possible to rank for content and on-page SEO only, backing up your onsite SEO with off-site work/link building can speed up the process of your website ranking.
I think if the content is useful, well-written and well-optimized, then the backlinks will come naturally. Yes, backlinks are really important, too, but I think it's better to focus mainly on creating useful content.
Backlinks and Content are just like two sides of the coins but equally important too. For the success of SEO, both are critical. But in the first order comes the Content and then the backlinks. And a similar theory that is applicable to both of them is having the best of both only matters.