Which is more efficient, towbar mounted, or roof rack mounted?

I'm guessing roof rack mounted is more efficient, but towbar mounted is more stable and easier to fasten the bikes to.

What do you mean "efficient"?

Fuel efficiency of the attached vehicle? If that's what you mean, then a rear mounted one would be a lot more aerodynamic than a roof mounted one.

Was going to ask the same question! I am thinking of getting a towball rack too as 2 or 3 bikes on the roof has a big impact on fuel economy, especially at motorway speeds.