Scuttles:
-) 6/6 is bigger than average Goyf (5/6)
-) Can't be chumpblocked by dorks (DRS, Stoneforge, unflipped Delvers)
-) Nugs for 6 on the way out
-) Deals
damage on its way out, which can be redirected to essentially vindicate any planeswalker
-) Colorless -- doesn't lose to Sword of X/Y, easy to cast
-)
You get to go "TOOT" every time you play it
-) Doesn't fly
-) Swords to Plowshares makes it sad
-) I can't think of any other drawbacks
Kokopuffs:
-) Has flying
-) Drains for 5 -- 10 point lifeswing
-) Doesn't target, so can't be stopped by Leyline of Sanctity (fringe)
-) Is double black, which is historically rough for tricolor Nic Fits -- especially if you want to Nightmare or Therapy it on the same turn an
-) Can't hit planeswalkers with his trigger
-) Can be bounced by Karakas (this can be an advantage in some circumstances, but in my experience is usually a downside)
-) Swords to Plowshares makes it sad
Summary: While the lifeswing on Kokusho is 'better', being able to essentially destroy target planeswalker I think gives the edge to Scuttles. There's a few very minor advantages and drawbacks to each, but that's the big one, along with the fact that Scuttles can't be Karakas'd.