I've used roof racks and hitch racks. They both work well.

My wife and I drove approximately 5,000 miles on this trip using the hitch rack, but we kept the roof racks on the car "just in case". We drove to the start of a tour, left our car at by brothers place, and did a 1,200 mile bike tour in southern Ontario. We had a lot of rain both driving and riding. This was the maiden voyage of the hitch rack and I was happy with it. However, the bikes get dirtier on the hitch rack. We cover the saddles on any type of rack to protect from sun and rain. IMO the roof rack is harder on the bike while driving in the rain. Water is hitting your bike at 70 mph, which is more than bearing seals are designed for. Hitch racks instructions specifically say, "don't use a cover on your bike". I've used bike bras on the roof racks and the generate a lot of resitance, enough to move my rack.