Yes, it is very tough to say which one is more important.
Somehow both SEO (Search Engine Optimization) & SMO (Social Media Optimization) are equal and they have their own importance.
In case if you want to make your user activity, or you want to convert your visitors into a customer. You can take the benefit of both SEO and SMO. In fact, both at the same time!! Debate on SMO and SEO is worthless to rank your website on Google.