When I was first looking for a puppy, I didn't have a strong preference one way or the other, but I DID want a dog who was easy and fun to train, as I wanted to do obedience and agility competitions with the dog. It so happened that the breeder that I really wanted a puppy from had a waiting list for girls, and then had a litter of all male puppies. After talking to a lot of different people, the consensus seemed to be that with this breed, there isn't any real difference between girls and boys in terms of "trainability". I was able to bump to the top of the waiting list because I was happy to take a boy!
As they grew, the breeder kept a close eye on them for me, knowing what I wanted in a dog. She was able to give me great guidance on which puppies would best meet my needs. Kodi has more than met my expectations. He earned his first Rally-O title at about 15 months, and now continues to compete in Rally as we prepare for Agility and formal obedience.
As other people have said, girls AND boys can be "humpers", though I am lucky that Kodi is not. He did it a few times before he was neutered, and completely stopped soon after. As far as marking is concerned some girls mark too. You just need to be on top of them and teach them that this is not acceptable in the house. I NEVER let Kodi near vertical surfaces where other dogs may have marked and he has never lifted his leg to pee.
As Becky said, 15-18 lbs is definitely on the large side, but certainly not the largest on the forum!!! You already said that you are going to neuter your pup, so you are obviously not looking for a conformation show or breeding dog. Kodi's parents are 11 and 12 lbs, respectively, and he is 16 1/2 lbs as a slim, trim adult. Who would have guessed?
I knew he was going to be on the large side... he was 6 lbs when I picked him up at 11 weeks. But he's a performance dog, not a conformation dog, and he's PERFECT for us. He couldn't possibly be a prettier dog, he's got good straight legs, good conformation and is sound and healthy. Best of all, he's also a sweet, friendly love-bug. I really don't care if he's a little larger than "ideal". We just call him our "lap and a half" dog!