Ceramic PCB is better. Due to ceramic board itself has pretty good insulating performance, which means that it doesn’t need an extra insulating layer so that the thermal conductivity of ceramic PCB is much better than aluminum PCB (metal core PCB).
PS: The insulating layer is the core technology of all metal boards and whose major functions are of bonding, insulation and heat conduction. Furthermore, the metal board insulation layer is the largest thermal barrier in the power module structure.