Are Sky Plate and Sharp Beak essentially the same in terms of boosting flying attacks? Are there any advantages (no matter how little) to having one over the other?
The plates and the type enhancers (Miracle Seed, Charcoal, Soft Sand, Poison Barb, Magnet, Mystic Water, Hard Stone, Sharp Beak, Never Melt Ice, Silver Powder, Spell Tag, Black Belt, Dragon Fang, Twisted Spoon, Black Glasses, Metal Coat; not sure what Fairy's is) are identical. The only advantage to there being multiple ones is to bypass the Item Clause, in a similar way to using both Black Sludge and Leftovers. Since the plates are more common, you could always use the Sharp Beak and hope that any opponent who finds it using Frisk has never heard of it before!
The boost is also identical to the one given by the Expert Belt, as well; they're all 20%.