Re: The best way to wreck my car!!
I've tried both.
Roof - structurally it's better, for the car and the bikes. Useful tip, turn handlebars 90 degrees and you get more space. I've got the swagon, so rails are easy. Previous car I had to clamp bars on and this wrecked the paint every time exwife slipped as we put it on. Lifting bikes on and off wasn't a real problem for me (6'3"), but even so, I did stick a brake lever into the roof once. So, refused to put bikes on the bella's roof. What to do?
Back mount rack. I was actually given one of these carriers - not an Alfa shaped one, but it does fit very well. Hangs on straps from the back of the swagon roof bars, with a pair of dumbells on straps inside the boot and a pair of straps down to the tow bar mount (it's a removeable ball type). Fits the back of my car perfectly. As long as the car is clean (and I do wash it before using the rack) there is no damage. Can only get three bikes on, but since I no longer need to carry the wife's bike, no problem!
Long and short, I'd go for the rack on the back. Amazed that I'd write that, since I thought they were total cr*p until I tried one. I think what gets them a bad name is people who don't mount them tightly/sensibly.
PS Anyone interested in a) Alfa supplied tow bar, now off car ready to sell it. All fittings, wiring, instructions for fitting bar and electrics.
b) Alfa supplied roof bars for swagon. Used twice, unmarked.
Probably going to EBay them, but thought I should mention it.