I am in the same boat, except that we have 8 adults.
My plan is to book whatever ends up being the best deal.
Right now, I have 2 minivans booked with
Budget, since I got a pretty great rate about a month ago.
But I also have two reservations with national, selecting from the emerald aisle in the hopes of two cars that will be better on mileage, but still have enough room for everyone.
I would advise figuring out how much luggage you anticipate having on the trip with you.
Then check out what size trunk space the different rental cars have and make your decision from there.
For example, I know that malibus and Chrysler 200s have really good trunk space, and we have crammed a lot of suitcases in them before!
Also, if the kids are in car seats, what will be easier to get them in and out of; car or minivan?
I would lean towards trying for a minivan and a car, giving plenty of room for the people and the luggage.
But if there is a substantial cost savings, I would not hesitate to book two cars with good trunk space.