E2E (End-to-End) testing validates entire user workflows, ensuring all parts of your application (frontend, backend, database) work together as expected. It is ideal and important for testing critical user journeys like login, checkout, or form submissions, especially when features span across multiple systems or parts of the application.

Unit tests, on the other hand, focus on individual functions or components in isolation and are faster and cheaper to run. They are sufficient for validating specific logic or component behavior without external dependencies.

E2E testing can be used to validate real-world scenarios and integration points, while unit tests should handle the majority of isolated functionality. A good test strategy balances both: unit tests for speed and coverage and E2E tests for user-focused validation.