Both of them are better since SEO is for organic traffic while SMO is for product or services branding.
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in my own point of view, smo really gives good amount of traffic in a very short of time when it is done correctly. in seo, it really takes time but once the keywords (must be competitive and related keywords) are positioned in search engines, the traffic is also great and it has 90% quality.
Social media sites help to generate targeted traffic, it's is good place for customer engagement, it works nicely once you start getting organic search engine traffic.
IMHO, it depends on your goals. if you are looking for good serp's then seo, but social media is a fantastic way of driving relevant traffic to your site. It allows you to get your website in front of your target audience and interact with your visitors on their level and bypass search engines. Both are important and a good mix of the 2 will go along way.