Truism #1: A horse CANNOT take the inside lead if his butt is to the outside. There are physical reasons for this, but as long as you remember to make sure that his body is slightly bent in the direction he is traveling, you will pick up the correct lead.
NB: “Bent” = from nose to tail the horse is curved around the inside leg. Too often people think their horse is bending correctly when only is neck is curved inward. WRONGO. The entire body must curve around the inside leg. This puts his haunches slightly inside his rib cage and he has to get the correct lead.
More often than not, a canter depart seems hard because the horse’s hind end is to the outside of the rest of his body. But if he is bent correctly, as long as you are consistent, it does not matter which leg you ask with. One reason to ask with the outside leg is that the leg also pushes the hind end to the inside. Still, horses or one-sided horses frequently cannot get their leads easily. This comes from their lack of suppleness in general and (often) has nothing to do with soundness.
BL: There is no right or wrong leg to ask with. What is right is establishing the bend of the horse. Dressage riders use the inside leg to push the ribcage slightly to the outside while H/J people use the outside leg to moves the hind end slightly to the inside.