Jacket then gilet on top, fairly obvious imo. It permits a layering option for colder weather early on and then removal of a layer, and then a layer on top again if the temperature drops. And it's easier to remove a gilet and store it in pockets/saddlebag. Please ignore replies from anyone who is a southern nobber and has no idea of what cold is. Also ignore replies from people who only ride "fast" or tell you that you are not riding hard enough as they are mostly full of crap.
I'm looking at you Rob