Here's my twopenneth worth. I have a LWB 61 Plate Citroen Relay and an 05 plate L200, so have both ends of the spectrum to judge from.
I love both for different reasons. The van is big and I have maximised the space by having racking etc. We do our window, fascia, gutter, driveway cleaning from this van.
We use the L200 for solar panel cleaning and a small amount of window cleaning. We have a 500L tank and two DI vessels in there and most of the time, one reel running from one pump. We fit an extreme 25' with the 35' extension in the back on top of the tank, wellies, bucket and other paraphernalia all in the back, no problem. Nothing needs to be on the back seats. We put the other extensions or other stuff when needed in the cab, but most of the time there is only the driver in the cab. On windows, the 500L tank will last two guys all day, but on solar it won't.
If I had one vehicle and kids, the L200 is perfect for everything. If I had one vehicle for all of the work we do, it would be the van. If I just needed a vehicle for window cleaning, I'd pick the L200.
The L200 is thirsty, but whatever vehicle you are driving as a window cleaner, you must be prepared for a hefty fuel bill. No one will have a van big enough for a 600L tank and only put a 200L in. We all run our vehicles to their maximum capacity, so will always carry a lot of weight. When done properly and it does take some planning, a 4x4 can look equally as professional as a van and can work just a s easily for window cleaning too. You are working with limited space though, so plan it well.
For your reference, although I put a 500L tank in mine, you can fit a 620L custom built tank in and the L200's are happy to carry the weight. They can carry over a tonne and tow loads too.
My tank is not bolted to the floor, so is not a modification. Structurally, should I have an accident, even at speed, the tank has 2 separate layers of bodywork with a gap between to break through before it reaches the driver, so is safe. The custom tank is crash tested and designed so that the top gives way and the water explodes out of the top of the tank instead of the momentum carrying the tank forward. Genius idea if you ask me.
Fire away if you have any other questions.