Ive used both roof and rear mounted carriers and can honestly advise you to get a roof carrier. They are much more secure, I drove about 2 miles with the tailgate carrier which I was advised was perfect for my car before turning around and returning it, the bikes bounced all over the place and honestly looked unsafe. That said since buying roof bars I can whack the bikies onto their carriers in minutes and the bars dont take too long to fit either and are much more secure when on, plus you can still use the boot.
Genuine VW bars pop up on eBay or in the classifieds all the time but I got some halfords ones which fit perfectly for the nice sum of £20.
