"Go for the hitch rack."
+100 The consequences of forgetting about your bike on the roof and driving under an overhang are dire; there is an insurance company ad currently on TV which shows the bikes, car and garage getting trashed by such forgetfulness. In addition I would not rely on suction cups to hold well enough to trust any of my bikes to them. If the harmless swaying bothers you there are straps at the auto parts store which would attach between the car and rack to stabilize it.