Let's take a look at the stats shall we
Slowbro
HP-95
Attack-75
Defense-110
Sp.Atk-100
Sp.Def-80
Speed-30
Slowking
HP-95
Attack-75
Defense-80
Sp.Atk-100
Sp.Def-110
Speed-30
Same stat base overall but Slowking has less Defense.
Let's see the movepool.
Slowbro
A rather deep and wide movepool.
Slowking
Deep movepool too but Slowbro has a deeper movepool.
So, it depends what you want, if you want a special attacker go with Slowking but if you want something bulky go with Slowbro.
Hope I helped!