Personally i prefer long boxes for the aerodynamics and therefore noise, the short wide boxes seem to be a good bit louder (and affect economy more).
By the same token, consider what you will put in there.
I always found a long thin box, way over towards my side of the car, was very easy to load and unload without a step ladder because i didn't have to reach to the middle of the car. It also meant i could put 2 bike carriers on the other side of the roof at the same time.
But if you want to stick a suitcase up there then measure the suitcase first, choose the roofbox later
