The Short Answer

Use PostgreSQL for SaaS apps with relational data and complex queries. Use MongoDB for content-heavy apps with flexible, document-shaped data. Use both when your SaaS has a CMS.

When We Choose PostgreSQL

  • Multi-tenant apps with complex JOIN queries across teams, users, and resources
  • Financial data where ACID guarantees are non-negotiable
  • Apps with heavy reporting needs (GROUP BY, window functions)
  • When Prisma is in the stack (best TypeScript ORM support)

Real example: Finflow — our finance SaaS case study — uses PostgreSQL with Prisma. The tenant → workspace → user → report relationship is deeply relational. MongoDB would have required painful manual joins in application code.

When We Choose MongoDB

  • Agency websites and CMS-backed content (blog posts, portfolio projects, case studies)
  • Apps where data shape varies per record (form builders, survey tools)
  • Prototypes that need to iterate schema fast without migrations

Real example: Bloom Co's e-commerce store — product catalogs with variable attributes (plants have different care requirements, pot sizes, SKUs) are a natural document fit.

The Hybrid Pattern

Our most scalable SaaS builds use both:

  • PostgreSQL (via Prisma) for: users, workspaces, billing, and any relational business logic
  • MongoDB (via Mongoose) for: CMS content, AI-generated outputs, activity logs, and any schemaless data

This isn't premature optimization — it's using the right tool for each data shape from the start.

Bottom Line

If you're unsure: start with PostgreSQL. Its stricter schema discipline catches bugs early and Prisma's type-safe query builder is the best developer experience in the Node.js ecosystem.