Roof racks on cars are great. They secure things outside the vehicle so they don't become flying missiles if you ever get into a crash. Much safer than putting unsecured luggage on your back seat.

On SUVs and Trucks though, I was always a little leery of roof mounted cargo carriers. SUVs already tend to have a pretty high center of gravity. I don't really want to be putting a lot of weight up high unless I absolutely have to. For those vehicle, I prefer the cargo carriers that secure to the hitch, low on the vehicle.

One thing to keep in mind is the GVWR/GCWR your vehicle is rated for. Most passenger cars and even a lot of unibody SUVs aren't rated to carry very much weight. A full load of passengers alone can actually put some vehicles overweight before you even start adding stuff in the trunk or on the roof. One must ensure that they are not overloading their vehicle, both for legal and for safety reasons.

For example, my Envoy is rated for a 6,000lb GVWR. The truck itself weighs 4900lbs as it sits. That means, I can't exceed 1,100lbs between passengers, cargo, and the weight on the trailer hitch.

That may sound like a lot, but if I have five passengers at 180lbs a piece, I'm already carrying 900lbs. That only leaves me 200lbs that I can divide between the trunk area, a roof mounted cargo carrier, and a hitch mounted cargo carrier or trailer tongue weight.

That's not a lot at all and we're talking about a vehicle that is built on a truck chassis with the heaviest duty components offered for a vehicle of that size. A car is going to be significantly less.