It is really up to you but I have found that typically, a spayed female and a neutered male get along best. But I am sure there are many who will tell you two females are fine as well. I have two spayed female dogs and people told me they would not get along well but they are best buddies. As for my cats, I have usually had a male and a female unless they were litter mates. That being said, Kittens tend to be much more accepting than adult cats, so your kitty will probably be okay with either sex
At that age it problaby doesn't matter that much if both are spayed/neutered. Ling is about 1 1/2 yrs old and I find that most females (after maturity) tend to accept a new male faster then another female - especially since they are the resident cat.
This is from experience with both spayed and unspayed females. I recommend getting a laid back male if the person already has a female. And even if the resident cat is male (neutered), they also seem to get along better when another neutered male comes in.
