These vans are unstable on the road when fully loaded and they tend to roll easily. Many, many people have been hurt or killed in rented large vans, and the insurance has gone through the roof. Many companies now refuse to insure them for rental, so the rental companies are dropping them. I'm surprised you got any quotes at all.
I'd say that if you've got two drivers, get two cars; it will save you a lot of money and give you more luggage space, and it's safer, too.