That’s up to you. The question you should be asking here is, “What do my marketing department or I do best, and what are the strengths of working with this agency?” After looking at your strengths, weaknesses, resources and capabilities, you may determine you can handle it on your own. Or you might decide that hiring a marketing agency can be an efficient, cost-effective route for producing value.
Being successful at inbound marketing requires a wide variety of complicated skill sets. You need to be able to plan strategy, write well, understand analytics, know buyer tendencies, design websites, create graphics and more. While you can hire internally, working with a marketing agency gives you access to all of these skills.