Usually cash gives you a better chance of getting some extras. It is unlikely to give you a discount as the dealer would much rather sell you the HP and get the commision. Remember HP means you do not own the van untill all payments have been made.
A bank loan allows you the same leeway as cash to the dealership, however you are paying interest on the loan. At least the van is yours from day 1.
Basically if you have the money available from your own sources, then use it as it will be the cheapest way to buy the van. If not, get a bank loan and only if all else fails use HP from th dealer.