GitHub Copilot is the established inline assistant — fast, in-editor completions across most IDEs and over a million developers. It shines at keeping you in flow: you write, it suggests the next line or block, and it rarely gets in the way. For day-to-day coding speed, it is hard to beat.
OpenAI Codex shifts from autocomplete to delegation — you hand it a described task and a repo, it works across multiple files in a sandbox, and returns a change set to review. For well-scoped tickets and refactors it saves real time, though vague tasks still need cleanup and everything needs code review before merge.