To me the difference is in the affordances and mental model.

Basecamp - groups all communication, assets, participants, tasks in a project space. I open the project which establishes context for everything and everyone. This predefined context makes short efficient communication very powerful.

It’s incredibly simple and sophisticated at the same time - allowing grouping activity by people projects tasks etc.

The ephemeral nature of chat-centered systems makes me incredibly nervous about missing things or being unable to find them.