All have a storage area in the belly of the bus and overhead storage. I have never been on a bus with spare seats. They usually have all of the seats full, and often much or all of the aisle. The shuttle to and from Cancun runs often enough that maybe they won't be so crowded. There is first class and second class.
Second class is better than a school bus and first class offers more room than an airplane.
Been on one first class and one second class that broke down. But I intend to travel by bus again in the future. Can't beat the price. Can anyone tell me the ave. The taxi drivers were asking 450 pesos.