From a business perspective, an app has some advantages derived from the fact that it runs directly in the device, as opposed to a website that needs an intermediate software to run (a browser).
It means that, with proper authorization from the user, your app can access features of the phone like gps and camera, but also the contact list and identity, that may be an invaluable characteristic if you want to implement a viralization strategy for instance.
An app also runs more smoothly, occupying the whole screen, while the browser shows the address bar and other controls that may disrupt the experience.
Advantages for the website include that with less investment you can target almost 100% of devices. If you want to do the same with an app, you'll have concerns regarding compatibility between different devices and OS versions and it'll cost you more to adapt your app to each platform, device and so on.
Good luck on the decision!