Since you are mobile, I wouldn't get an air powered one.
Few things that I do not know.
1. Do you have your own Generator?
2. Do you have your own water supply?
3. Do you have a Pressure Washer?
If you are talking about the Foam Cannon, I have one and use it just about every detail with the pressure washer. It may add 5 minutes to a detail, but I feel it has it's benefits. To me, it feels like my mitt slides easier over the vehicle (more lube) and it helps dissolve dirt as the foam is siting on the vehicle.
I buy my soap 5 gallons at a time. I don't think it wastes too much soap. As far as hooking it up, mine is pretty simple. I have quick disconnect which makes going from the foamer to the pressure washer tip very simple.
See pic:
If you are talking about the one that hooks up to the water hose, then I don't know cause I haven't used one of those.
Stay away from the the air one if you are mobile.