I would say that what you go for depends largely on your work.
All my work is within a half mile radius of where I live and much of it is close together so my trolley comes out of my car and stays out 90% of the time. If I were to work only from a van mount I'd be constantly moving the van, and parking where I need to would be real tricky. (One comedian friend of mine suggested getting a half mile hose and working from my back garden.)
I find somewhere central to where I'm working for the day or morning and park there. Then I haven't got to go far to get refill during the day.
But that's my round. Yours may be different.
As for working from a car with it, some trolleys are bulky and you'd be lucky to even get them in a car without problems. I can get the new Freedom trolley and six 25ltr barrels in the trunk of my Nissan primera. I have a large plastic box with lid for my trad gear on the rear seat. I curently use two poles. A 12'x3 section which goes in the car with ease and an 18'x3 section which goes on the roof but which can go in but not with ease. The pic shows the trolly and 5 barrels but two are laying down to prevent them toppling. But stand them up and I can fit another in.
It is a good bit of kit and well built and it suits my work down to the ground. The rest of the kit works well too.