Short answer

What to know

Choose httpbin to test methods, headers, authentication, redirects, delays, compression and status codes. Choose Payloads when you need stable domain-shaped data, exact checksums or equivalent text and binary representations.

The short comparison

NeedPayloadshttpbin
Inspect your outgoing requestNoYes
Generate status codes or redirectsNoYes
Test delays and compressionNoYes
Known users, projects and tasksYesNo
Equivalent cross-format datasetsYesLimited format response helpers
Manifest and checksumsYesNo

Choose httpbin for protocol behaviour

httpbin describes itself as an HTTP request and response service. Its endpoint catalogue covers methods, authentication, request inspection, response headers, redirects, delays, compression, streaming and status codes.

Choose Payloads for data behaviour

Payloads keeps the HTTP behaviour intentionally boring: successful static GET responses with correct media types and CORS. That isolates decoding, validation, importing and file handling from a programmable server.

Primary reference