Choose MongoDB Atlas when you want a document database and a free shared tier to start. Choose PlanetScale when you want a monthly cluster price for Postgres or Vitess and you do not need a permanent free database.
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