I personally think in regards to how competitive siblings are it really does come down a lot to personality, parenting and lots of other factors other then gender. I don't think that girls are inherently more competitve then boys (in my personal experiences it is more often the opposite or in a more overt way at least).
I was 1 of 4 sisters with a brother bang smack in the middle of us.
For me I have always wanted both boys and girls. I liked the romantic idea of the big brother who watches out for his little sister/s. Also coming from a girl heavy family (dh family is also girl heavey) a boy always seemed so much less attainable. I never doubted I would have girls (even before being pregnant) but feared that I would never have a son.