Rear will offer better fuel economy and virtually no likelihood of impact with low ceilings.
Roof racks will offer somewhat better security (as the bikes are just a little bit harder to reach), however I have seen instances where the racks themselves were stolen off the car (with bikes still attached), resulting in damage to the roof of the car.
Since your topic asked: the lightest system
and probably the best (as far as I've witnessed) - http://www.seasucker.tv/
Not sure what the actual weight of the rack is.
|| Other projects in the works.