I’ve used brass, aluminum, carbon fiber, and polycarbonate plates, here are my thoughts on each:
My favorite, extremely rigid, makes for great sound amplification, and a firm bottom-out. Some might find the bottom-out feel to be too harsh though. It’s also fairly expensive compared to the other offerings.
Goldilocks’ plate of choice, not poor, but also not the greatest (personally), it’s fairly attainable/inexpensive and has the same properties as brass, only I find aluminum to be slightly softer and doesn’t give as large a difference to sound signature. Otherwise, it’s the safest and most vanilla choice of plate.
A fairly malleable plate material, it’s fairly soft, moreso than aluminum, but has the sound signature of polycarbonate. Carbon fiber is also much lighter than metal plates. The bottom-out feel is also comparable to plastics. Carbon fiber is around the same price as aluminum and polycarbonate so it’s fairly inexpensive as well.
My least favorite plate material, very very malleable, extremely soft and brittle, and doesn’t give a satisfying sound nor feel. Arguably, the soft characteristics of polycarbonate make a nice typing experience, but building with polycarbonate plates with how bendy they get, and the sound results they convey in a fully-built board, would rather not choose this plate material. It’s cheap for a reason.