For most feedback, I'd always recommend posting the feedback here on the forums because it allows other users to chime in with their ideas and +1s. We always share forum feedback with the appropriate eng team internally.
However, in the case of the new IDE, it's a new feature so the product team created that Discord channel to get user feedback right away without having to wait for myself and my teammates to relay it from forum posts.
In other words, always post feedback here unless it pertains to a beta testing channel in Discord, in which case you can just post it directly in Discord. Even if you do add it to a beta testing channel in Discord, it doesn't hurt to add it to the forum since it'll give more visibility to other developers 
Update: Our Discord is now retired. Find us here on the Community Forum, on r/retool , or join us live at Office Hours .