It's been my experience that when in a buffet anywhere, the servers have a much larger group of tables that they are serving versus a plated restaurant where the waiter may have only a few tables they serve. The difference is, the buffet waiter does a walk through, picking up plates than a walk through, filling water glasses. The plated restaurant server must spend much more time & detail per table bringing different courses, suggesting different pairings, etc. Also, clients at buffets typically do not occupy the table as long as clients at plated meals so the server does not usually spend the same amount of time with them.
I am not saying that one works harder than the other but if the buffet server has a larger group, it is because that person's work is less detail orientated.
Buffet servers we tip around 10%. Other waiters, we tip between 15-25% depending on the quality of the service.
I think at the end of the night, both servers would probably make about the same when multiplying it by the amount of tables they would have served.