Generally speaking i don't think it matters which mix of sexes you have.
We had Alfie for over a year before we got our 1st girl Jo Jo....sadly he couldn't abide her for the 1st year or so
then as the months & years went by, both of their feelings changed & now they absolutely adore each other
Alfie is now nearly 7 & Jo Jo is 5.5 years old. Next into the gang came Nina who is nearly 3.5 years old....her & Jo Jo get along fine & Alfie accepts her as one of the pack. So far then 2 girls & 1 boy.....then just over a month ago we had another addition.....a black puppy, Georgia, which of course has changed the dynamics again. Alfie, Jo Jo & Nina have all accepted her straight into their group
I won't pretend that everything is perfect all the time & there can be a little upset each time a new dog is added but a lot depends on being aware of any potential conflict before it happens & dealing with it (in hopefully) a positive way.
e.g Nina does guard toys/high value food so we only allow toys with Georgia when she is either in the puppy pen or if Nina is in a seperate room etc....apart from that though things seem pretty cool atm
I know a few COL'ers that have 2 or more girls so if you & your family are 100% committed to having another dog for the whole of his/her's life & accepting that it may not be all 'hunky dory' between both dogs (although they might be best buddies from day 1) & if you would prefer to have another girl imo i would go for it.
Let us know what you decide.