I would pay the extra and get the towbar system, even if it means extra expense, for these reasons:
- On my wife's Touran 4 bikes on the roof reduces mpg from around 47mpg to less than 35mpg.
- A towbar mounted system can be used on the spur of the moment if you fancy going for a bike ride at some local forest or other track. Roof mounted bars and cycle carriers take much longer to fit when needed and hence you won't bother doing spur of the moment stuff
- I'm a shortarse and so lifting the bikes up on and off the roof is challenging - I have to stand on the sill and then have someone behind me to stop me overbalancing backwards, or dropping the bike on the roof. I use a stepladder at home, but don't have that option at the destination because the car tends to be too full of luggage to take a stepladder with you. All this may not be an issue if you are tall, but it is still more difficult than a towbar mounted system.