Structs offer compile-time safety, better performance, IDE support, and method attachment for known fields. Maps are for dynamic or unknown keys at runtime.
Field names are known at compile time — compile-time typo safety
You need to attach methods and implement interfaces
Performance matters — struct field access is direct memory offset, map access involves hashing
Working with JSON APIs — struct tags control serialization cleanly
You want IDE autocompletion and refactoring support
Keys are only known at runtime (user-supplied, config-driven)
Sparse data where most fields would be zero-valued
Implementing caches, lookup tables, or frequency counters
Processing arbitrary JSON blobs of unknown schema
Building dynamic query parameters or HTTP header collections