Based on your facts: Both have same functionality and assuming you have done your research with users and they do not have a clear preference.

I would highly suggest starting off with a web app. Here are the reasons:

1. You can get exposure to a website (e.g., inbound marketing, SEM) with more ease compared to a mobile app. You can have a website for a mobile app as well, but then you have to encourage them to go to the app.
2. The product development for a website is typically easier. Example: You can fix bugs on the fly, deploy as many times as you want and when you want. With a mobile app, you need to go through Apple review (for iOS). Yes - there are options with hot code push, but it is not mainstream.
3. Apps usage has saturated. Most people use less than 5 apps (http://fortune.com/2015/09/24/apps-smartphone-facebook/). So even if you get someone to find your app and download it. Getting them to use is a separate story.

A mobile enabled website can do the trick. Especially, if you are in the product-market fit stage, I encourage you to try the web platform first.