Different design goals
FIX is widely associated with institutional trading connectivity and stateful message flows, while REST is commonly used for request-response application integrations. Actual capabilities depend on the provider specification.
Trading connectivity
For execution and market-data integrations, use the protocol and message model supported by the counterparty and appropriate to the required workflow.
Business applications
CRM, client-area and administrative integrations often benefit from conventional application APIs where request-response semantics fit the workflow.
State and recovery
FIX session sequence and recovery behavior differs from stateless HTTP-style APIs. Both designs still require idempotency, reconciliation and error handling.
Security
Protect credentials, restrict network access where appropriate, rotate secrets and use least-privilege authorization.
Observability
Track errors, latency and dependency health using protocol-specific metrics while preserving end-to-end correlation across workflows.
Architecture choice
Choose per workflow. A broker stack can legitimately use FIX for institutional connectivity and REST-style APIs for customer or back-office applications.