Once you try a roof rack ... you won't be going back to anything else IMO. I've been using a Yakima roof rack since 1992 and it is the best thing since sliced bread (errr ... Pizza too!)
IMO -- roof racks are safer (for the bike and car), more stable, and provide piece of mind.
+1 on the core locks and wind deflector.
__________________
-\Brian
18' Landshark Tandem - Custom
15' Wabi Special Single-Speed Road
06' Cannondale Six13 TeamOne
06' Cannondale Prophet 3 "Lefty"
92' Trek 5200