Overall, Clefable is the superior pick in the current Smogon OU metagame. Being able to sponge attacks from both spectrums, abuse great support moves, and 2 arguably better abilities, there is a reason Clefable is S Rank in OU at the moment. There are various sets that it can use, such as Stealth Rock Support set, Mono Attacking Calm Mind with Thunder Wave, CM + 2 Attacks, cleric, or even Life Orb 3 Attacks / Full out Attacker.
Sylveon, while possesing a greater immediate offensive presence, thanks to its Ability Pixelate, it gains access to a powerful STAB Hyper Voice that goes through Subs, making it great versus some set up sweepers. Sylveon is most known for being a prime reciever for Baton Pass boosts (although that has been nerfed yet again by Smogon), but also has its trademark Cleric set and its powerful, although less durable, Specs set.
The pros outweigh the cons. Clefable is a great status sponge (Magic Guard), blanket check switch in (takes no damage from Hazards with Magic Guard), or even a great way to cheese your opponent after they've set up some boosts and find out it was in vain (Unaware). If you want to be a devil about it, Mono Attacking Calm Mind set with Unaware can be used to set up ON set up sweepers, although you forfeit a reliable healing method (Soft-Boiled) and a support move (usually TWave) in exchange for Wish + Protect.
Sylveon only has the immediate power that Choice Specs brings, competing with Mega Gardevoir in terms of raw power and difficulty to switch in if you're not a Specially Defensive Skarmory or Mega Scizor.
CLEFABLE > SYLVEON
Note:
In Doubles, while I may not be as well versed in the format as others, Sylveon is the superior choice, thanks to Hyper Voice being a dual hitting move (like Rock Slide), working absolute wonders with its ability Pixelate.
