LGDs work best in teams. The best combination is actually a team with at least one boy and one girl - their strengths compliment each other. With most LGD breeds, the females usually tend to be more energetic and are often the first ones to chase down the predator. However, the males, although often low key, do an excellent job of finishing off predators. One thing to note - male dogs that are not neutered can be more aggressive and not as easy to handle.
We currently have a male Pyrenees/Anatolian mix who is so calm, some people would view him as lazy or lethargic. However, he is awesome at keeping predators away, not just from our goats (he's fenced in with the goat herd), but also our chickens and guineas who are on the other side of the fence.
If you choose to only have one LGD, it's probably just your personal preference, but if you're getting a puppy, I would encourage you to get them in person from the breeder and observe their behavior with their litter mates. Sometimes whether they're male or female doesn't matter as much as the individual dog's personality.