For the story, who is better, and which moveset and item?
Always go Delta Scyther. Being Ice/Fighting, speed is a lot more important than bulk, so Delta Scizor isn’t usually helpful over Delta Scyther. Get one with Hustle, train it in speed and attack, and it’ll wreck a ridiculous number of mons while still having enough bulk to take at least one neutral hit most of the time. Icicle Crash, Close Combat, Knock Off, Hone Claws, and Ice Shard are all good options. I’d stick with the first three and then decide the last move on what you need it for (Hone Claws sweeps, but Ice Shard lets it revenge kill and avoid being murdered by weakened faster pokemon like Mewtwo and Alakazam). Hone Claws sets like Focus Sash, but you could also use Eviolite, Leftovers, or Life Orb. Ice Shard likes more power or speed: Choice Band/Scarf, Life Orb, Expert Belt, Metronome, etc.
You can also replace anything that isn’t Close Combat or Icicle Crash for a Rock move (Rock Slide, Rock Tomb, Stone Edge) or Iron Head. Stone Edge would be great with Hone Claws, but you don’t get the TM until really late postgame.
Edit: changed Night Slash to Knock Off, added other move options.
So, Delta Scyther?
My team is: Eevee: Flygon (Mega); Electivire; Delta Haxorus; Tyranitar (possible armor); Delta Scyther