Worked with wfp system for years; my experience is that it all depends
on what type of customer you have (more commercial or domestic) and
how far you have to travel.
If you have plenty commercial a trailer system if ideal (lots of water!) plenty
of space by customer, if more domestic, then think of the streets and parking problems
with a trailer; can you always and easily backout the street ?.
Trailers have the advantage of being parked at home if not needed, but probably
you will throught the system have énought work to eventualy afford another vehicle
for private use.
A van system is much quicker to get on the job, quick parking, door open and off on
the job.
Perhaps if you are unsure you could buy a trailer system and when you realise
that a van would be better have your trialer system put into the van and sell the trailer.