I actually think NoSQL might be better for MVP and prototyping. Why? Because they can handle churn more quickly, and you don't have to worry about scale.

Once the data layer starts to resonate around a solution ... then pick the right horse for it.

Why not just use a SQL db that handles json and stick everything in a couple columns? Changing databases is a lot of work, and as far as I can tell, mongo isn’t really providing much value over SQL dbs that already support schemaless json columns.

Because you still have to write a layer between your web app and the database at the prototyping stage?

I can't just make calls from Html + JS to Postgres.

But I can with MongoDB

Or you can use something like Postgrest: http://postgrest.org