I have two of each. Two sisters and two brothers. I hardly talk to one sister (her choice) and the same with one brother (My choice). I get along well with my older sister and the youngest of my older brothers. The oldest brother, well, lets just say I am less than civil with him most of the time....
I don't think the sex of a sibbling really matters. Girls enjoy playing together and boy do too. And when they're young, boys and girls play well together. As they get older they go thru awkward stages and become estranged and such, but as they grow up (20's and settling down etc)they often grow together again.
I think it depends more on individual personality. There are always going to be clashes and the sex of the child doesn't determine this. What I am trying to say (and not very well I must admitt) is it's kinda six of one and half a dozen of the other! *LOL* I think ur children will be happy no matter what the sex of the next bub is! Goodluck!