Have you used Firebase before? In my (rather limited) experience, the development speed increases from not needing to manage your own Mongo store, not to mention implementing all of their real-time features on top of that, are quite valuable. At least for an MVP I wouldn't hesitate to do the first version with Firebase to test my assumption even if I knew I wasn't going to use it at scale. Plus you can always wrap Firebase in your own layer from the beginning.