Different breeds have different temperaments. I would suggest finding one of the many free kittens in almost all areas, since there are so many, a lot of people struggle getting rid of kittens.
Boys aren't usually as affectionate (although they can be very affectionate), and sometimes like to rule the roost. They are sometimes more aggressive, and if not fixed, once they reach maturity, they spray. Every where. And it smells. They are also prone to cat fights if they come across cats outside. On the other hand, they are more independent and less demanding. They can handle themselves.
Girls are calmer, after kittenhood of course (cause no kitten is calm), and usually more affectionate and attention needy. They also tend to be more fearful, so strangers and loud people (kids especially) are likely to get a swat.
Depending on whether you have frequent visitors, or not, and how much time you and your girlfriend are going to put into the cat, should conclude which gender. A lot of breeders and vets suggest getting two related kittens of the same gender to keep the kittens behaved and preoccupied, problem with that is it costs more; but not by much.
Its always cheaper to get boys, when it comes to veterinarian care,and you should fix both genders no matter which you get. Females can get pregnant, males pee all over. If you get them fixed at a young age, that will never be a problem.
:3 Good luck on teh kitteh hunt, it won't be a mistake! Kitties are lotsa fun and energetic playful critters.