For “simple” PETG, i.e. not loaded with other materials. A brass nozzle is a good choice to keep the best performance, or better yet, a Copper-Nickel nozzle (more resistant).
A hardened steel nozzle is much less efficient (thermally) than a brass or copper nozzle, but becomes mandatory in case of abrasive material. Abrasive materials loaded with glass or carbon fiber for example.
For example, a brass nozzle will wear out completely in a few hours with an abrasive material… But for any non-abrasive material, it will give better results and allow less heating. This limits the degradation of the material.