Male vs. Female
Many people believe that female dogs make better pets...female preferences seem to be ingrained in these people. Most calls for puppies are people wanting a "sweet girl". They don't think females display alpha behaviors like "marking" and/or "humping". They believe that they are more docile and attentive and do not participate in fighting over dominance.
In the dog pack makeup, females usually rule the roost, determine pecking order, and who compete to maintain and/or alter that order. The females are, as a result, more independent, stubborn and territorial that their male counterparts. The females are much more intent upon exercising their dominance by participating in alpha behaviors such as "humping". There IS a reason people use the technical dog term "bitch" in a negative way - and it refers directly to the behaviors exhibited by the females of the dog world. Most fights will usually break out between two females. Males, on the other hand, are usually more affectionate, exuberant, attentive, and more demanding of attention. They are very attached to their people. They also tend to be more steadfast, reliable and less moody. They are more outgoing, more accepting of other pets, and take quicker to children. Most boys are easily motivated by food and praise, and so eager to please that training is easy. However, males can be more easily distracted during training, as males like to play so often. And no matter what age, he is more likely to act silly and more puppy-like, always wanting to play games. Boys are fun loving until the day they die. Females tend to be more reserved or dignified as they age. Witness the human equivalent of the twinkling eyed Grandpa still playing catch at age 70, while Grandma quietly observes from the porch.
Boys do get bigger than girls. It's hard to exactly pinpoint the weight ratio between males and females because genetics does play a big role in how big a puppy will grow to be.
Neutered males rarely exhibit secondary sexual behavior such as "humping", or "marking" and lifting of legs. Once the testosterone levels recede after neutering, most of the behaviors (if they ever existed) will disappear. Boys who were neutered early (by five months of age) usually don't ever raise their leg to urinate.
And while the female will usually come to you for attention, when she's had enough, she will move away. Boys are always waiting for your attention and near at hand. Females are usually less distracted during training, as she is more eager to get it over with, and get back to her comfy spot on the couch. The female is less likely to wage a dominance battle with YOU, but she can be cunning and resourceful in getting her own way. She is much more prone to mood swings. One day she may be sweet and affectionate, the next day reserved and withdrawn, or even grumpy. The female also has periods of being "in heat", unless she is spayed. Seasonal heats can be a month long nightmare, not just for the female, but you and every male dog in the neighborhood. If you are not breeding, you'd be better off to have her spayed, since during this time she can leave a bloody discharge on carpets, couches, or anywhere she goes. She will be particularly moody and emotional during this time. A walk outside during this period can become hazardous if male dogs are in the vicinity, and she will leave a "scent" for wandering intact males to follow right to your yard, where they will hang out and "wait".
Before deciding on male or female, give consideration to any other dogs that maybe in or around your home.