I like Slowking more. I do not deny Slowbro's battling ability. It's superb and I know it. But it's the design I care about here. Slowbro is Slowpoke with a sheel to the very last extent. Boosted Stats, same appearance, and same intelligence. Rip off the Shellder and you've got yourself a full-fledged Slowpoke =/.
Slowking, however, has a greater change to its appearance, and a very cool one at that. It's also arguably one of the smartest Pokémon ever, in high contrast to its brother(pun intended

). It may not be as effective, but it qoulc certainly be a lot more fun to hang around with, ne?
Plus, Slowbro=Kanto so I'm naturally biased against it >=D
~*CB*~
The 8th Champion