Module usage description - I couldn’t describe better
In other words: if you need not only back-end CRUDs with views, but used in back-end only models and/or components and/or external libraries, then, of course, you’d better to use a separate admin module.
IMO, there’s not that big difference for a simple back-end between these approaches, and the choice often depends on whole application design and architect developer taste.
If you want to go for advance you should go with module or add sub-folder "admin" in /controllers and /views and put you controllers and view files into them…