There's not a whole lot of difference in size. The Grand Prix does have little more room inside & in the trunk. If you don't have huge amt. of luggage and only 4 people, it's not worth it to pay the extra. Even if you have a lot of luggage the trunk of GP is not that big. (We owned one and also have had G6 as rental) Some of the other full size cars have pretty good size trunk. If the difference is only a little and you think you need the room, go with the larger car, but it also uses more gas.