650 tank everytime.
doesn't even come close to payload as 650ltr includes baffles, actual volume of water is about 600ltr (I checked mine on the landfil weighbridge with empty tank and then with full tank, same amount of fuel in van too (full tank)
with 2 reels, 2 pumps, full load of water and 4 poles (no ladders) and just me in the van I was still 108kg under the gvw (gross vehicle weight)
so I could easily fit someone else in the van with me (as I do).
Don't forget that as soon as you start in the morning the total weight inside the van is going down all the time.