I've been using them in combination and that has been a real boost to me! Co-pilot is great for ideas. Sometimes I want an explanation of what something is doing or to ask about methods for accomplishing a certain task etc. I'm just using ChatGPT in the web browser at the moment, but in the IDE itself would be fantastic, though at the moment I don't feel confident about giving an API key to a 3rd party extension that I don't fully trust.