Anyone can read your content. Only your team members can edit it.
A public space can be read by everyone, and will be indexed by search engines. You still have control over who can edit your content (including seeing your drafts). This gives you the most freedom and increase the visibility of your content.
Unlisted spaces won't be indexed by search engines such as Google. They will still be accessible to anyone who link to your documentation. It lets you easily share your work-in-progress without it being searchable by your customers.