What is the difference between a Sidekick and Spaces? When would I choose one over the other?
Sidekick is a general AI assistant. Its job is to solve problems. That means it will default to solving problems and giving answers (which is excellent if that's what you want it to do). On the flip side, if you don't want it to provide answers, you need to tell it not to explicitly.
Create a Space will only do what you ask it to do (sort of the flip side of Sidekick), so it won't just inherently give answers to anything it's asked unless it fits within the prompting you give it.
For example, creating a sidekick and calling it a "science tutor" will solve any science problem a student asks. It's also going to give answers to anything else as well.
Create a Space will redirect the student to the topic in the AI prompting and won't let the student get off task.
The short version? Sidekick's job is to give answers and support. Create a Space will only do what you tell it to do.