I've a 400 litre upright at the moment,.. swapping it for a 650 layflat over the holidays.
I do like upright tanks, but you really have to know your stuff when securing them in the van. Even the new layflat tank will be in a cage and bolted through the chassis,.. but I'll feel a lot more comfortable with that behind me than the upright,... even if it does take up half my van.