When it comes to SEO and increasing traffic, you must make sure you have these two things down, as off-page and on-page SEO can increase your conversion rates. Both of them are important.
Both are instrumental to a comprehensive SEO strategy, but I think at the very beginning it's more important to focus on on-page and content-centered SEO because that's easy to integrate and can have a long-term impact.
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If I had to choose beween the 2 I would bet on Page Seo. But agree with the rest of respondents that these 2 fields work in synergy. If you neglect either of them the other one will be of zero use, I guess. That is why I try to use every seo tool out there available for a specialist in the field. At the moment I am studying https://rush-analytics.com/land/... from Rush Analytics. Amazing set of tools!