Boy or Girl Puppy ?
I think gender differences become unimportant when Yorkies are
spayed or neutered.
I have raised, trained and loved both female and male Yorkies. I honestly think behavior is more a personality thing, than a gender thing.
Right now I have four Yorkies, two of each. My boys are more physically affectionate. The boys are less demanding about being the top doggie and let the girls boss them.
Abba Daddy is the sweetest most gentle Yorkie I have ever had. Rubin is ten years old and kind of set in his ways. He prefers people play to doggie interaction. For the most part Rubin is tolerant of fosters and has only had one fight in all these years.
My girls are Gracie who was a rescue and came with many personality hang ups and Ginny Joy who I’ve had since she was a puppy. Gracie is the boss and more moody than the boys. She also will bite hard and nip the other dogs if they bother her. She is my little selfish diva. I love her 'had a bad start but, look at me now' attitude. Ginny Joy is very sweet, not as kissy as the boys. She loves brushing and mothering toys or foster puppies.
So to answer the original question, look for personality, spay or neuter after sexual maturity* and don't worry about picking a boy or a girl. Love makes the perfect match.
* I have read a lot lately about waiting to
spay and neuter until a dog is sexually mature. Several studies conclude it is better for bones, teeth, organs and personality development. My Vet has convinced me it is a real advantage to wait until Yorkies are at least 1 year of age.