Celesteela and Skarmory DO NOT compare. Skarmory has access to hazard, Defog support, Whirlwind, Roost, a powerful Flying-type STAB and Counter, to name a few of its gems. Celesteela, in comparison, has no hazard, no hazard removal, no phazing move, no reliable recovery outside of the hazardous Leech Seed and Leftovers, and finally the weak Air Slash, in fact so weak that Flamethrower is the better choice.
This is not to say Celesteela is inferior to Skarmory, but to say they do not compare. If I said Celesteela sucks, I'd be lying. Celesteela has much stats and distribution than Skarmory, it has some nifty tools, an AMAZING ability and excellent stalling potential.
Celesteela surpasses Skarmory as a stalling Pokemon, but it fails to phaze out enemies, remove hazards, sustain itself reliably, place said hazards or even work as an absolute physical wall. Skarmory looks more specialist, but again, it lacks Celesteela's better overall bulk, movepool and stats.