Whisper API: a) Timecodes; b) how good is open-source vs API?

Revised (see below for how to get timecode from the api), the advantage of using the API over self hosting is performance and model size, if you wish to run a very capable model from a low power device, offloading that computational task to a remote, enterprise class compute facility is an attractive option. If you have local compute of sufficient power then local use is fine.