Depending on how much time you'll be devoting to the digital marketing program and tasks like content creation, our preference as an agency is to have a client that has a dedicated internal marketing person to support the communication and feedback that we need as an agency.

If you're at a budget level where it's either one or the other (in-house vs agency), then I think the decision needs to be based upon the level of time you have to dedicate to the program. If you have a lot of time to spare (more than 15 hrs per month) then hiring an outside agency is a good way to divvy up the workload and have the agency help you prioritize strategy and what you're working on.

If you don't have time to dedicate, then the decision should be based upon your expertise to manage an internal person. If you have a basic SEO skill level, and you can't afford a full-time senior SEO role, then you'll probably want to bring in an agency to make sure someone with the proper expertise is prioritizing efforts for the fastest results.

Happy to chat if you have questions on the decision process.

-Kane Jamison
http://www.contentharmony.com