Clearly wizards would smoke, because nothing is quite so dignified or contemplative then smoking, nor anything so quintessentially magical. I think they would smoke tobacco or pipeweed, though you could make a case for cannabis. But witches, possesing a very different magical framework/persona/paradigm, would they smoke? Historically it was unseemly for ladies to smoke, but would a witch wish to flout these cultural norms? Or maybe, being closer to the healing magic of the elements witches would know how destructive smoking is? Just shoot out any ideas really, it's all free game here.