SEO (Search Engine Optimization) and SMO (Social Media Optimization) both are terms of digital marketing fields. SEO improve a website presence in organic search engine results. So SEO is the process of improving the visibility of your website to your online audience via search engines.
SMO increases the visibility of your business and brand on social media profiles. Social networking activity and your published content so that it is found by people searching for information and resources that relate to your content. Social media optimization helps drive traffic to your website or blog, makes it easier for others to share with their circle of influence, builds the community around your brand and helps search engines index your social media profiles, social activity, and blog posts more easily.
So I preferred always SEO for better results for the website.
It depends on your requirements, because if you wants to get good ranking and organic traffic from search engines then SEO is the best option. And if you wants to promote your products and services or for branding of your products then SMO is the good option for you to reach from a lot of people in without cost.
There's no way I can control social media websites. Yeah, I can submit stuff and network till I'm blue in the face, which is OK. But SEO is something I can control, I have 100% control over my website, to do on-site SEO and can also control where I get links from and how to best promote. I never ditch any possible traffic generation tactic, so I try to pay attention to both of these.
It's not either or question though. If you want to reach more people, you need both of them. I tend to spend more resources and time on social media though. They have more traffic than SEO efforts can give you.