The PostgreSQL vs MongoDB decision is made by the project's data shape; there is no general "one is better" answer. We've used both in production (MongoDB in BASH CHAT, relational structure in the quote system); our decision guide is below.
When PostgreSQL?
When relations are clear and rule-based: e-commerce (order-customer-product), finance, reporting-heavy work. ACID guarantees, JOINs and a strong type system shrink error margins. Full-text search and JSONB also add flexibility.
When MongoDB?
When document shape varies: messaging, catalogs, log/event data, fast prototypes. Schema evolution happens without migrations; horizontal scaling is natural. But relational queries and transactions are weaker.
Performance Facts
Both are fast with the right indexes. MongoDB shines in write-heavy streams, PostgreSQL in complex reads/reports. Wrong indexing hurts more than wrong choice.
Using Both Together
In big systems both live together: transactional data in PostgreSQL, document/stream data in MongoDB. Healthiest is starting with one database and adding the second as needed.
Backup and Maintenance
Automatic backups + restore tests are mandatory in both. If you've never tested a restore, you have no backup.
FAQ
Which for a new project? Start with PostgreSQL for most SME web work; consider MongoDB when message/event weight dominates.
Do you offer database design? Yes — within custom software.