Magician is much more common. "Conjurer" is more likely used in a fantasy fiction. If you want to be technical: A magician is someone that uses magic. A conjurer is someone that conjures things. Conjure means, "to make something appear from nothing." Often, magician is a term used for illusionists, or modern people who use trickery to make things appear like magic. Often, they will make things appear from nothing, like a coin coming out of your ear or a rabbit from a hat. Those acts are called "conjuring."