Both are effective but SMO is updated than the SEO...SEO is a tradition thing, but the SMO is new arrival thing, which helps more effective to getting quality backlinks with good traffic as early as possible. when compare to SEO..SEO is a late process but its constant process i.e creates organic traffic.
SMO is one of the part of SEO. SMO make a better traffic for site.SEO has a big concept and it has many method for site optimization like SMO,Link Building, Off-Page, On-Page, Black Hat, White Hat and many other. So SMO is a sub part of SEO and SEM. So do not confuse with those concept.