Which Hostile Mob will spawn in a dark room?

In a dark room with the requirements met to be able for hostile mobs to spawn, is the spawning of different types of hostile mobs (etc, Creeper, Skeleton, Spider, Enderman, Zombie) random or is there a preference for a specific hostile mob to spawn?

Assuming that the correct spawn conditions are present, all passive mobs have the same chance to spawn, and the 4 basic aggressive mobs (spider, skeleton, zombie and creeper) have an equal chance to spawn. Mobs that spawn more rarely are: Endermen, Wolves and (while not really having a chance, to spawn, they still are encountered more rarely) Silverfish.

The rarest of all mobs is the Spider Jockey, which is a Skeleton riding a Spider, every time a spider spawns, there is a 1% chance it will spawn as a Spider Jockey.