The debate is still open and been going on for years.

There is only 2 advantages for a sinking popper that I know of.

First the extra weight give longer casting distance. Since a popper is a cast and retrieve lure the longer the cast the better.

Second, a sinking popper allows for very fast retrieve. If you retrieve a floating popper too fast it will bounce off the water into the air. A sinking popper will stay in the water even with very fast retrieves.