Round are considerably better than square. Square tend to “localise” any flex at their corners and unless very well supported internally, will fracture along those corners. Always try not to weld aluminium; particularly if it is going to be subjected to continuous flex and twist, it will fracture along the weld in a relatively short period of time. Always fasten beams by drilling and bolting or pop riveting, always use compression spacers with the bolts so that the load is not localised just where the hull meets the bottom of the beam or else the bolt can tear through the beam. There is a very good reason why the aluminium skin of an aeroplane is fastened with pop rivets and not welded (although now some do use resin adhesives instead)
Oh and by the way there are many agricultural vehicles that DO use square drive shafts.