Honestly Thunder, it really won't make a difference. Besides, once they're spayed/neutered, you actually have an "it" cat.
Also, sometimes male/female pairings can be better in some cases, as there won't be the "alpha male" wars.
I've had male cats, female cats, etc. and it really depends upon the personality of the cat, not it's sex. My female kitty is extremely affectionate...she doesn't play much anymore b/c she's about 11 years old, but she's very loving.
And since they're getting along so well right now, why would you upset the apple cart based on experience you've had with different species?