I prefer email, becuase it's easier than trying to catch a time when BOTH parties are able to actually concentrate on just the communications. With email, it's read when the receiver has the time to devote to it, rather than picking up the phone when you're in the middle of something else.

I know that some do this for a living, and thus maybe they keep "business hours" but you could be working in the snake room, arranging eggs in the incubator.. who knows?

But the main reason for me would also be the paper trail showing exactly what any agreements are.