This is a great question and an important one as well. I would start by identifying:

Who is your target market?
How will they use your product? On the go? On their laptop or on their mobile device?
What kind of technology does your product need? Can it be built as a simple website or does in need to be an app (i.e. you need push notifications, integration with other mobile apps, etc).

On the other hand, how much time and budget do you have? Typically a responsive website is faster and cheaper to build than an app and will get you out the door faster.