Plastic, why? because Bronze is....... well, Bronze age.
The good transducers like Airmar 744s in Bronze have a plastic fairing block encasing them anyway.
I chose bronze on my wooden boat. The plastic is not so tolerant of expansion, so I am told, and can also become brittle over time. The fairing block is plastic with a bit of teak to get it right but I think the key element is where it passes through the hull. With a GRP hull things may have different priorities.