I think it may come down to amoongus having more recovery options and higher hp and clefable's abilities being better and having a better move pool. Unaware and Magic Guard could be quite helpful. I'm not a big competitive battler so I don't have the best advice. I'd probably go with amoongus, myself, but I'm partial to grass types.
Hmmmm. I'm not sure what to tell you here. Both have a semi-reliable recovery move, Moonlight or Synthesis, although amoongus also has access to Ingrain and Giga Drain. Clefable's Magic Guard ability is probably gonna be more helpful than amoongus's Effect Spore. What are you thinking of pairing it with?