For the bulky set, Impish nature gives Mega Scizor 3 more Attack and Defense, while targeting same Special Defense stat. I know this is a bit of nit picking, but always use nature that takes advantage of Pokemon's main stats. The outcome is clear in this case. :3
Bulky Swords Dance
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name: Bulky Swords Dance
move 1: Swords Dance
move 2: Roost
move 3: Bullet Punch
move 4: Knock Off / Superpower / U-turn
ability: Light Metal
item: Scizorite
evs: 248 HP / 84 Atk / 176 SpD
nature: Impish
EDIT: There's another thing I don't get with the physical EV investment though. Adamant nature with 8 Def EVs is enough to prevent Mega Scizor from getting 2HKO by both non-Life Orb Excadrill and Lanodorus-T. Both the bulky Adamant and Impish nature set will faint in 3HKO regardless. So doesn't that make Adamant nature superior then? Unless there are other unmentioned Pokemon other than Excadrill and Landorus-T that hasn't been brought up.