females are best imo, they stay very close to home and are naturally protective. also, if you want a dog that's gonna challenge those bigger labs and get respect pronto, it's gonna be a bitch
you're right in that many folks pick a puppy when they find one that's attracted to them, I've selected pups by clapping my hand and seeing which one ques in fastest on the sound and source, and repeating several times.
best of luck selecting.......