Should I choose powder coating or hot dip galvanising for something outdoors?

It depends on how exposed the item is and how much colour matters. Powder coating gives any RAL colour and a clean finish, but it is a barrier: once it is chipped, corrosion can start at the damage. Hot dip galvanising to EN ISO 1461 protects sacrificially, so a scratch does not undercut the coating, but the finish is matt grey. For exposed items where colour also matters, the answer is often both: galvanise first, then powder coat over it. That combination has to be decided before fabrication, because galvanising requires vent and drain holes in closed sections.