In my experience, a male would fit in better. I've always felt that females tend to dominate more often than males, although this is not always the case. I have 3 males and a female and she is alpha. 1 male is the oldest of them all, and the other two males are younger than her.
Opie dogs aren't human, they don't have best friends. females deffinitly don't have best friends
I agree with the other posters a male and female don't dominate the same, they usually just glide into the role without as many fights. If she's fixed you don't have the heat problem and thats pretty much the only problem between male and female.
If you think dogs can't count, try putting three dog biscuits in your pocket and then giving Fido only two of them.
I would suggest male! I have a spayed female Boston and just got a second puppy. I went with a male and she and he get along well together. Although he seems to want to be the Alpha (although he is half her size) It is kinda funny to watch them.