This article is from the Getting A Cat FAQ, posted to rec.pets.cats newsgroup. Maintained by Cindy Tittle Moore with numerous contributions by others.
Neutered males and spayed females make equally good companions. Although some people insist on cats of one sex or the other, cats actually vary in personality independently of their sex. Neither sex is uniformly more affectionate, more intelligent, more calm, or more playful.
Unaltered cats of either sex, however, can be difficult to live with. Unneutered males "spray" a foul smelling urine on the walls and furniture. If allowed outdoors, they will roam and fight with other cats. Unspayed females may also spray, and usually "call" when they are in heat; this is an incessant yowling that will drive you and your neighbors to despair! Neutered and spayed cats make much more pleasant companions.