SEO is a process of improving the volume and quality of traffic to a website through search engines.

When it comes to SEM-search engine marketing it is concerned with promoting a website, increasing profits,increasing traffic etc .

So we can conclude that SEO is the subset of SEM and no SEM campaign is complete without SEO.

As both are almost same you can use any one,but I suggest you to use SEO as it completely deals with backlinks so you can get high pagerank.