Slowbro and Slowking have identical stats, excpet their Defense and Special Defense stats are swapped around. This means that their Surfs will be equally strong. Slowbro has the higher Defense, which I think is more useful than having a high Special Defense (as Slowbro and Slowking's only common physical weakness is Shadow Ball, and you could argue Megahorn, whilst they have a plethora of common special weaknesses such as Thunderbolt, Crunch, Giga Drain, Hidden Power Electric, Hidden Power Grass, Solarbeam, Thunder, ThunderPunch, etc.)
So I'd choose Slowbro, but if your team is very weak, in terms of Special Defense, you may have a bit of an excuse to use Slowking (although I think a Chansey or Blissey would be a better option).