Travelling from Arusha to Dar by bus, two of the companies with good records of safe drive are the Dar Express Bus Serviceand Kilimanjaro Bus Service. These companies leave according to the schedule and drive in a recommended speed. They also have Lunch spot at Segera for few minutes before continuing to either Dar or to Arusha. Their travelling schedules are not the same.
The rest of the information you can get on their booking offices.