I have both. Although the trailer is only 5'x3', I can can carry 8'x4' sheets on it with a small overhang at the front and a larger one at the back. I have had a 5.2m joist (for my conservatory) on the roof bars and 5m lengths of aluminium (1 tube, 2 bar for building a hi-fi stand). I have also transported 12'3" lengths of galvanised, box-section, roof sheet, supported by the two halves of an extension ladder on the roof bars.
My father once took an extension ladder to France (to work on their holiday home) - the sections slung side by sides *under* the roof bars ... to keep under the height limit and avoid having to pay extra.