I believe they stopped making the Grand Marquis in 2011, but you will still see them around. As far as the 300, it's a great car, and I get over 30 MPG on the highway with it, so am always pleased to get one as a rental. From my experience the 300 is the largest of the Premium cars at Budget, unless you get one of the older Grand Marquis. If you call Budget they will give you a very long list of cars that are considered Premium, and some are not large cars.

You might call the SLC station and see what cars they have as Premium. You might call the manager a few days before your pickup date and tell him/her that you need a car with a large trunk and see what he suggests with what is available. I have been offered an SUV at the counter at no additional charge when none of the Premium cars sounded that good to me.

I have received as premium cars, Ford Taurus, Grand Marquis, Altimas and the 300. Many of the premium cars are what I would consider full size, but they usually have more of the options, leather seats, better stereo, etc to make them premium.

For room, you might consider one of the larger SUV's so the two adults in the back seat won't think they are still seated in the coach cabin.