A lot of people think a girl is softer and gentler then a boy. This might be true in nature, where the tomboys have to be tough fighting for territory, mating partners and food. But with our cats getting sterilized as young as 4 months, they never reached hormonal maturity and thus never felt the urge to mark territory (spray furniture and walls) and to impress a rival with ear-
Female cats also benefit from early sterilization. Apart from losing the urge to look for a mating partner (which shows in the longing to go outside, in rolling around and crying for partners), they become more balanced and relaxed. Females never lose the need to have their own territory (this is the reason why it is more difficult to add another cat to an existing female than to an existing male).
Therefore, if boy or girl is not a big question anymore. Go for what personality talks to you the most.