What are the main differences between SSJS and AMPscript, and when would you choose each?

AMPscript is a lightweight markup language designed for email personalisation. It is optimised for high-volume send-time rendering, has lower server overhead, and is the only scripting language supported inside email sends. Use AMPscript for email personalisation, DE lookups at send time, and simple conditional content.
SSJS (Server-Side JavaScript) runs on the Marketing Cloud application server. It supports modern JavaScript syntax, complex data structures, HTTP calls, and the full WSProxy/Core library. SSJS is available in CloudPages, Script Activities in Automation Studio, and landing pages, but not inside email sends. Use SSJS when you need API calls, complex logic, error handling, or cross-object data manipulation that exceeds AMPscript capabilities.