My partner has metal plates and I have the polycarbonate style. Both black with white writing. Hers have seemed to fade and oxidized badly, tho we have never really polished them in 10yrs. Mine started to fade after a year or so, but I have found this product (
GOLD CLASS TRIM DETAILER | Meguiar's ) to really work well on the polycarbonate. Have only used it twice on my plates and they have never looked so clean.
If you go the metal ones, at least it will be cheaper to replace if you happen to damage one of them.