What's exactly the main differences or similarities or pros\cons with each of these 2 options 'just 'having fun' 'testing code' 'using the app locally by myself without shipping it'
Codex if your priority is fast iteration, building from scratch, and staying in a conversational workflow.
Claude Code if you expect to spend a lot of time improving, extending, or maintaining larger projects.
If you eventually build products with thousands of lines of code, you'll likely find value in using both depending on the task.