Although Slowking is my favorite of the two, Slowbro, competitivley, is the more widely used one because it is simply better(Not my Opinion, but a fact)
Having higher Defense than Special Defense helps alot in the more Physically-oriented Metagame. The only thing that Slowing can do that Slowbro can is Nasty Plot. But even then, having Lower Defense and Higher Special Defense hurts it more than most people think.