Is it better to hire a media agency to write a story board for my corporate video?

You should be the one setting all the information and ideas you need to deliver to the audience as well as the main goal of the video.
When you decide on the above mentioned details you will be ready to work with a media agency or media experts in order to help you set the right video and guide you.

The advice of an expert is always recommended in order to ensure that the final product is just perfect.
It is very important though that the company defines very clearly what the values are and that need to be communicated in the video as well as setting the core objective that needs to be delivered.

It’s not a must.
Audience these days want to feel that your corporate message is something that really reflects your companies ethics, beliefs and environment.
I think it’s a good idea to prepare your story board outlines on your own and then have a professional agency take care of the production details.

I advice to do so to end up with a professional work.