I have both. The roof one is total secure it even locks onto the frame to deter the passing theif. the towbar one is also very secure once all the straps are done up tight, but I have to provide my own lock, and additional lightboard with number plate, also I can't use the towbar mount when towing the trailer.
The tow bar mount didn't effect the mpg (in fact without the trailer on the back I could drive at proper motorway speed which incrased my mpg due to the cars gearing). I'm not sure about the roof effecting mpg as I'm normally towing a 1 tonne trailer which does effect mpg.