Both are equally good. At the same time, skills required and organizational structure for both areas would be quite different. For e.g., SEO is more resource intensive compared to SEM. For the same reason, it's relatively easy to scale SEM operations compared to SEO. Also, the stakes involved in SEM are higher due to budgets involved.
In my view, at some point of time, for an organization to grow, it would need to offer both the services as there's a lot of synergy between SEO & SEM and clients would like one organization to take care of both aspects.