I have both a hitch (with a 4x8 folding trailer and bike rack) and roof rack (setup for two bikes and a canoe). I don't have and won't use one of those cargo carriers that go in a hitch. The forces are just all wrong for the hitch, it is only a matter of time before it breaks.
Any hitch for your car will likely take a 1.25" square tongue, any off the carriers I have seen need a 2" tongue. Can you even get a cargo carrier that fits your car?
Roof Rack
Advantages
Better for long things (ie. windsurf boards)
Cheap to convert to new vehicle (new qclips cost me $60)
Convenience, most summers it lives on the car so it's always with me.
Disadvantages
Cost (buy used and the price becomes reasonable, mine was ~$850 new, bought used for $200 and then bought the Qclips for $100)
Fuel Economy (bikes on rack are the worst, just plain rack is ~5 percent down)
Limited weight capacity (most cars are rated for about 75 lbs on the roof, the rack weighs half that) Obviously most people ignore this and load the crap out of it.
To carry small things, you need additional parts (bag/box). The boxes cost a fortune, the bags scratch your roof.
Hitch
Advantages
Can pull trailers when you need to move lots of stuff (or really big stuff)
I normally have just a tongue/ball in to inflict damage on those that suck at braking
Bikes on the back aren't crushed when you forget about them and drive into the garage.
Disadvantages
On a normal day, it doesn't add any cargo capacity
Adds zero to resale value of car (maybe subtracts), can't be used on next car
If you have some mechanical inclination (or know someone who does) order your hitch from hitchdepotusa.com They want $180 for your hitch with tongue and ball and $20 for the harness (in case you want to pull a trailer). I ordered hitches for 2 vehicles from them for ~$400 and the install took less than an hour to do both. No customs BS, the price in the cart was the price I paid.
http://hitchdepotusa.com/Lookup/rear/2009/Pontiac/G6/Sedan
If I only had one system, I would say the hitch and harbour freight trailer is the best combination.