I don't bother with roof-mounted carriers... PITA to lift a muddy bike onto the roof.
I use a hitch-mount platform carrier -- the Sportrack 2EZ (or Xport Flatbed 2, the Performance Bike version). I'd love to get a Thule T2 but cannot justify the extortionate price.
Stay away from hitch-mount carriers that support the bike by its top tube IMO, especially if you have a full-suspension rig.