I you launch a new product, business.. etc and you want to get traffic fast then SMM best because it is use for increase the traffic of your website and popular your products, business, brand and website using Social Networking websites like Facebook, Twitter, LinkedIn..etc. These social networking websites have huge number of users on their website, when you join them and post your links on their relevant posts, they will help to increase the traffic of your website.
If you want to get traffic for long time then SEO is best because in SEO, you post your links on other websites and they keep your links for long time, from this they help to provide the traffic for long time.