Well I would pick a male. I have 6 cats; three are female and three are male.
My boycats are BY FAR friendlier, more human-loving, and outgoing. They have the 'best' personalities. They constantly want to be around us, whereas the girlcats hide. Dusty is very very very picky; she's loving and all, but only to certain people and she is just weird towards the other cats. KC only likes to be petted while it's dark and in my bed only (very antisocial). Miss Jessie we hardly ever see; she only comes out maybe 5 minutes a day; once again, an antisocial girl cat.
I vote for male cats! Mine are of course all neutered and all different ages; Hammie's 3, Zorro's 6, and Snickers is 13. However even with the difference in breeds of my boys and where they came from and the age differences? They are all so very sweet, loving, and just loaded with personality!!!
Since you have a female -- and she is used to being an "only child"... maybe judge by her personality as to the type of cat you get. If she is shy and sort of aloof, then maybe a cat who is outgoing & friendly would be good, to help pull her out of her anti-socialness. If you get a male cat to be her sibling then make sure he's neutered or there will be problems! Either way judge by the personalities of your cat & the new cat.
Out of my 6 cats, I only chose 3 myself. I picked 2 of the girls, and a boy. The other girlcat was a stray & the two boys were strays, and they adopted ME.