Your question is dependent on whether or not you are an authorized dealer for a particular brand. If you are, the answer is obvious, yes. If you are not, then the question becomes more involved. There are many plus sides to have 1 brand: mechanic training, parts inventory, etc.
The downside is your cost of new equipment and replacement parts.. There is much flexibility in the pricing of new trucks and parts and if there is no competition when you puchase a new truck or parts, the pricing could be higher.
As long as you have some control of your equipment and parts pricing, using one brand does have an overall advantage.