I have both and use them both depending on the situation. Through the winter I always use the roof mounted ones and or throw the bike in the boot. I find the bike is more protected from the salty gunk when on the roof so when you arrive to go biking your brakes actually work.
In the summer it depends on how far I am traveling and if the roof rack is on the car or not but generally I prefer the tow bar rack.
Advantages of the tow bar rack is the speed in which it can be attached to the car. 2 bikes can easily be mounted as you just leave the middle of 3 spaces empty, meaning the bikes don’t touch. 3 bikes can be a total pain especially with full sus frames with shocks in the way etc. I find the tow bar rack also significantly reduces fuel consumption too, just not as much as the roof.