I'd go with the minivan. It's the most reasonable way to get 6 people and their stuff around. A big SUV will suck more gas and not really be any more comfortable.

Ostensibly you could seat 6 people into some premium or luxury cars, like a Grand Marquis or Town Car, but when you book these classes there's no guarantee that's the car you get. If you book minivan you'll always get a minivan that seats 7.

The Mazda 5 is a sort of mini-minivan that seats 6, but there wouldn't be room for any luggage, and there's no way of knowing when you would get this vehicle. (It is in some Hertz fleets at least, but the class varies and there's no way to book it specifically.) Similarly, some smaller to mid-size SUVs have third row options, but these are not guaranteed to be configured that way, and there's no way to book it that way.

The minivan is the most economical too, gets better gas milage than a jumbo SUV.

Another option as mentioned would be two cars, which in some cases could price out cheaper than one minivan. Assuming you have two drivers, that is.