Well, if you're talking about the "soft" roof top carriers versus the hard plastic ones, I can give you my opinion.
We started out with a soft carrier that we could easily roll up for storage in our basement when not in use. We used it ONE time. The noise was deafening - like hail beating down on the roof!!
For our next long distance trip, we did not want to use it because of the noise and also the hassle of getting it firmly attached up top. (It had straps that you had to wrap around the roof rails and tighten.) We ended up leaving stuff at home and cramming the rest into our van.
After that, we bought a hard carrier from Sears and have been very, very pleased with it. The only down side that I can see is it does take up more room when being stored in our garage.