Both are great. Actually the free version of ChatGPT, 3.5, is good enough for most cases. Note that ChatGPT is 'offline' though, you need to copy-paste code back into your IDE.
Copilot is one notch above because it integrates directly into your IDE. If you follow coding channels on Youtube, you will notice a lot of creators use it for auto-fill, that's where I first saw the power of Copilot and am using it now.
I've used this one as well, but in terms of functionality, isn't it similar to GitHub Copilot? I find that GitHub Copilot in general works better and seems to understand context a bit better than the ChatGPT + VS code plugin.
Copilot is one notch above because it integrates directly into your IDE. If you follow coding channels on Youtube, you will notice a lot of creators use it for auto-fill, that's where I first saw the power of Copilot and am using it now.