The hitch mounted cargo carrier is a wonderful device. I've used it many times. For luggage, for firewood, for mulch, for a hot water tank. Very little hit to fuel economy. The roof mounted carrier I would never do. It's ugly IMO, it affects aerodynamics, it can be noisy, and it's no fun to try to heave and hoist a 70-80 lb suitcase up high like that. Chances of hurting yourself or scratching the car.