In my personal opinion i feel like girls are preferable.
I've had both...the leg hiking is what i don't like. All my males have marked,even after neuter so after they passed away i had to replace all my furniture and drapes(all ruined 4 inches up!) Not to say i dislike boys...i don't!! I just don't like to deal with all the penis issues!
ALL yorkies are fabulous (IMO) You can't go wrong either way!