It depends, If you are looking for quick business in minimum time then SEM is the best technique to generate instant business and on the other hand social media marketing is a way to promote your site services and increase your brand awareness in targeted audience.
SEM mean of search engine marketing. This is use for promote websites by increasing their visibility in search engine result page and SMM is Social Media Marketing which includes promotion of websites through Social Bookmarking, networking websites, forums posting, blog and article posting, and many more.