I've got the Thule T2 918XTR and the add on for 4 bikes love it this is one that secures the wheel which is nice if your anal about your paint on the bike you will never have scuff marks with this type.  and I've had other roof racks from Thule their customer service is excellent I had a rear tire clamp break called customer service had 1 in the mail 3 days later no questions no charge just told them what broke and they sent a replacement