Skid, trailer, or truck mount for a two-man crew.
The format decision you live with every day for five years, and the tradeoffs nobody mentions at purchase.
Skid
Drops into the bed and stays there. Setup at the job is pulling hose. The truck goes where the truck goes, which is the whole advantage and the whole limitation — parking, drive-throughs, and residential streets are all easier.
The cost is bed space. If you also need to carry a surface cleaner, chemicals, and a second person's gear, it gets tight fast.
Trailer
More room, easier to load, and you can drop it and take the truck. Bigger tanks mean fewer refills, which on a route matters more than it sounds.
The cost is maneuvering and parking, plus a second registration and the insurance conversation that comes with it. Backing a trailer into a tight commercial lot at 6am is a real skill, and not everyone on your crew has it.
Truck mount
The most integrated and the most expensive. It makes sense when the rig is the business and the truck exists to carry it. It makes no sense when you also need the truck to be a truck.
How to actually decide
Think about the worst-access job you took last year, not the average one. That job is what determines your format, because average jobs work with anything.
And think about breakdown time at the end of the day. Fifteen minutes of extra packing up, five days a week, is sixty hours a year — which is real money and, more to the point, it is the thing that makes people quit doing the work carefully.