If Chu Xin had been given the choice, he would have preferred Slowpoke to evolve into Slowbro. Still, Slowking was no bad choice either, with no real drawbacks compared with Slowpoke.
Compared with Slowbro, this branched evolution introduced in Generation II had far less presence. In fact, many beginners thought Slowbro was its unevolved form, and occasionally someone would even run an Eviolite Slowbro. Then, after Slowbro gained Mega Evolution, Slowking, supposedly possessed of great wisdom, never learned to Mega Evolve. What a joke. Their base stats differed only in their defenses: Slowbro took physical hits better, while Slowking resisted special attacks better. In practice, though, both had solid defenses on either side and could comfortably serve as mixed walls. Water/Psychic simply lent itself better to physical walling, so players in OU preferred Slowbro over Slowking. Still, Slowking was hardly much worse, especially with its absurd special bulk after putting on an Assault Vest. Its Abilities were exactly the same as Slowbro's. Regenerator was naturally the usual choice, while Oblivious could be used to avoid Taunt when setting up Trick Room.
There was no fundamental difference between Slowking and Slowbro as walls, except that Slowking was better suited to an Assault Vest, while Slowbro more often carried a Rocky Helmet. Slowking's natural special bulk was already extremely high. With maximum HP investment and an Assault Vest, even most super-effective STAB moves could not take it down in one hit. For example, a Life Orb Tapu Koko's Thunderbolt did at most 85%, a fully invested Serperior's Leaf Storm only had a chance to deal over half, and a Choice Specs Hydreigon's Dark Pulse did no more than 70%. It might not compare with Chansey, but that special bulk combined with Regenerator let it pivot into most special attacks without much trouble.