Choose MongoDB Atlas when the data is documents and an M0 cluster is enough to prototype. Choose DigitalOcean Managed Postgres when you want a 1 GiB Postgres node at a flat $15.15/mo.
MongoDB Atlas
Managed document database with a free shared tier
Free M0, then paid
Strengths
M0 is a real free cluster for prototypes
Dedicated tiers cover production
Limits
A real bill splits compute, storage, and backup
Document modeling is a poor fit for relational workloads