I'd go with bronze. It's more susceptible to electrolysis, but is also more forgiving of flex. I once installed a Marelon fitting in a place where are the thickness of the hull was uneven over the diameter of the hole. When I torqued the backing nut it cracked the side of the mushroom of where the hole was thicker. Up until then I was a big fan of Maralon. Now, not so much.