Aider is more of a collaborative chat, where you work with the LLM interactively asking for a sequence of changes to your git repo. The changes can be non-trivial, modifying a group of files in a coordinated way.
Workspaces seems more agentic. You need to do a bunch of up-front work to (fully) specify the requirements. Even with a perfectly formulated request, agents often go down wrong paths and waste a lot of time and token costs doing the wrong thing.
That's also not how I code personally. My process is usually more iterative.
Another big difference compared to Workspaces is that aider is primarily a CLI tool. Although I just released an experimental browser UI [1] yesterday, making it more approachable for folks who are not fully comfortable on the command line.