The main advantage is not foisting unsigned on the user of the API.

(You can do that while using unsigned internally, but then you have to convert back and forth.)

The most important decision is what is the index type at the API level of the circular buffer, not what is inside it. But it's nicer if you can just use the API one inside.

The sizeof operator yielding the type size_t which is unsigned has done a lot of harm. Particularly the way it spread throughout the C library. Why do we have size_t being unsigned? Because on small systems, where we have 16 bit sizes, signed means limiting to 32767 bytes, which is a problem. In all other ways, it's a downer. Whenever you mention sizeof, you have unsigned arithmetic creeping into the calculation.

The author of the above blog article has the right idea to want a sizeof operator that yields ptrdiff_t instead of size_t. (Unfortunately, the execution is bungled; he redefined a language keyword as a macro, and on top of that didn't wrap the macro expansion in parentheses, even.)