I was excited when I bought my box truck and I'm sure it will serve me well. However I'm nearly certain my next purchase will be a butler unit in a van. Simplicity + Reliability + Resale Value + Space Savings = clear winner in my book. Especially when moving towards a multi-truck operation it's important to have a system that is easy for new employees to figure out and use. That's not to say it can't or hasn't been done with slide-in truckmounts in big box trucks either because it has. I just believe that when building a brand, it's important to create consistent systems that are universal throughout the company.
On a side note, I would consider having a one of those powerful dual-wand capable machines mounted to a cabover box truck for very large commercial jobs.