I think the answer is 6" - this may be 1" per seat or a wider aisle. Does not sound a lot, but two consecutive sectors in economy on a 747 (10 abreast) and a 777 (6" narrower but 9 abreast) will convince you it is.
The bus generally has wider seats than the equivalent Boeing: try the window seat on a 737 compared to an A320 to really appreciate the difference.