Well, here’s the Pros and Cons of each
Slowbro
Pros:
•Superior stats to Clef
•Regenerator is an amazing ability
•Access to reliable recovery in Slack Off, Scald with a burn chance, Future Sight, and Calm Mind for setup.
•Can work as a good trick room setter
Cons:
•Not as good a movepool as Clef
•5 weaknesses and no immunities
•It’s a bulky water type, so it faces stiff competition from a certain crown-of-thorns starfish Water/Poison type in the tier
Clefable
Pros:
•Magic Gaurd and Unaware are both amazing abilities. Magic Gaurd prevents any sort of chip damage, while Unaware annoys setup sweeprs.
•Fairy is an extremely valuable typing, beating the common Fighting, Dark, and Dragons, and gives it an immunity to dragon.
•One of the most versatile movepools of any Pokémon. It has Stealth Rock, Soft-Boiled, Wish, Teleport, Moonblast, and Calm Mind at it’s disposal.
•It’s rated S tier lol
Cons:
•Not-so-good stats
•Not many resistances
Overall, I would prefer Clefable for it’s awesome movepool, ability, and Fairy typing being valuable in OU.
Now for the Movesets:
Clefable @ Leftovers
Ability:Magic Gaurd/Unaware
Nature:Bold
EVs:252 HP/252 Def/4 SpA
•Stealth Rock
•Soft Boiled
•Calm Mind
•Moonblast
Slowbro @ Heavy-Duty Boots
Ability:Regenerator
Nature:Bold
EVs:252 HP/252 Def/4 SpA
•Slack Off
•Teleport
•Future Sight
•Scald
Hope this helps!