Tiers don't matter!!!
Choose whichever one suits your need more. But I would go for Sylveon as a cleric simply beacuse he can pass bigger wishes with a higher base HP than Sylveon. Even if you say Support Cleric Sylvy is still superior because of its access to Reflect.
[email protected] Clay/Leftovers
EVs - 252/0/4/0/252/0
Ability - Pixilate
I would use Light Clay over Lefties because Sylveon can heal up using its own wish, or if you are going with Lefties, then choose Heal Bell over Protect.