Consider your competition. Look at what your competitors are doing and how they are performing in their search marketing before you decide how you can best compete with them. Research what search terms they rank organically for. Consider if you can execute a plan to top their SERP placements. Also, look at what paid terms they are using to drive traffic to their own sites. As you perform this research, look for gaps that you can fill and areas where you will be unable to compete in both paid and organic search.
Ever heard of Maslow's hierarchy of needs? It's a theory of psychology that prioritizes the most fundamental human needs (like air, water, and physical safety) over more advanced needs (like esteem and social belonging). The theory is that you can't achieve the needs at the top without ensuring the more fundamental needs are met first. Love doesn't matter if you don't have food.
Additionally, there are many situations where PPC (a component of SEM) makes more sense than SEO. For example, if you are first launching a site and you want immediate visibility, it is a good idea to create a PPC campaign because it takes less time than SEO, but it would be unwise to strictly work with PPC and not even touch search engine optimization.