I think that either another male or a female would be good. It all depends on the cats personality. You don't want 2 cats that want to be the alpha or top cat because then you'll have fighting problems and this can also lead to spraying or territorial problems.

From my own experience, I've found that by adopting a younger cat then it's less of a threat to the original cat but you don't always know a younger cats true personality like you would an older cat.

The main thing to also keep in mind is to introduce them slowly and to always make sure that your original cat still gets lots of attention. I'm sure that you'll do just fine. Good luck.