is there much to choose between different styles/makes/models?

Ford S-Max; thinking of having a longer thinner roof box and 2 bikes on top.

Was going to get towbar-mounted carrier, but have to get towbar too, and with car full we appear to need a roofbox for holiday anyway….. Or is the towbar mounted carrier that much better?

opinions welcome. Cost not critical but as cheap as possible is always good – but since bikes are £800 and £3k (carbon), want something decent enough to do the job.

what would you recommend, and where from?