In some cases, the higher class bus may use the expensive and faster toll road, while lower classes may use the slower free road. The price is always very reasonable, usually around 2 or 3 dollars US per hour. The price difference between classes is typically less than the cost of a beer, so I look for comfort and convenience. I use both First and Plus class buses, and avoid second class buses except for short distances or if there is no other alternative. They may put your name on the ticket.
Buses usually leave on time.
Most ticket agents probably do not speak English, so look up the correct phrase or write it down before you get to the window. I usually buy my ticket at the station, but sometimes it is more convenient to buy a ticket from a travel agent. However, a travel agent will only represent a few of the first or plus class lines. The choices will be wider at the bus station. I generally buy my tickets for First or Plus class buses a day or two before, and request seats near the front, preferably the first row on the right hand side, which are the best seats for sightseeing.