In my experience with different masts, the alu ones are A LOT heavier.
Stiffness is not much of an issue for the freerider, and they are both quite stiff, unless those carbon ones made in a "less optimized" layout (and there ARE differences, also in price).
So if you have alu and carbon masts almost same weight, and tmcfarla even one where alu is lighter than his carbon, I would keep the alu.
Havent experienced this myself though, quite opposite, the weight of alumasts is one of the things that put me off.
You are right though, that hydrofoils with alumasts are also often with higher density wings thus heavier in the water, and the fuselage could also be heavier alu, so it is not only the mast that weighs.