I changed my mind.
I found a good quality hard shell rooftop luggage carrier through Craigslist for $50. It more than doubles our minivan's storage space. I put it on the roof this past weekend to road test it. No noise whatsoever.
We will take a hit on the mileage, but a previous poster noted that they were saving a ton of money by driving and that another $40 in gas would be worth it for the comfort of the passengers.
Having the carrier will allow the kids to put pillows and blankets in the back when they don't need them.