I had a Saris bones on a hire car once (own car written off, not my fault, couldn't use car for planned trip as it had no tow bar, insurance co. paid for a strap on rack on proviso I sent it to them when I was done) I didn't like it at all. I felt the need to check and tighten the straps every hour or so, and it still felt only semi-attached. In comparison I drove to Bordeaux and back with my own tow bar mounted rack and didn't worry about the bikes at all.
A tow bar needn't be too expensive if you get an indie specialist to fit it - I got mine fitted for £60 by a guy who is a mobile tow bar fitter. You do need to declare it as a 'mod' to the insurance, although it shouldn't affect the cost.