Thule are great. If I could afford it I would go for the towbar mounted options as I think this would be more convenient. It is easier for loading, and if yo want to shoot off biking on the spur of the moment it would be so much quicker to do so. However it is more expensive and there is the issue of how to hide the towbar away when not needed.
The other aspect is that roof mounted will damage economy. On my wife's car (VW Touran) 4 bikes on the roof reduces economy from 47mpg to < 35mpg.