Are plastic or ceramic sockets better, or is there any difference?
The material is significant. Thermoset plastics are what are usually used for sockets. The black-brown stuff most are made from is “bakelite” a trade name for a kind of clay-reinforced phenolic. Maybe there’s a variety that is purer or more sturdy phenolic that is more resistant – I’m fuzzy on that one. All plastic sockets are vulnerable to arcing. When you get enough voltage from pin to pin on an output tube to cause a spark to jump from pin to pin (like when you run the output transformer unloaded) the spark runs along the surface of the socket material and burns a trail on the surface. Since the plastics contain carbon, there is often a carbon residue left on the surface. This residue is partially conductive, and makes that path susceptible to arcing over at lower voltages next time; this can be so bad that it interferes with normal operation. Ceramics are not carbon based, do not burn in the normal sense, and don’t soften or melt at temperatures achieved in an arc over, so they are
