head to head
Neither is simply harder - they are hard in different places. Call of Duty: Black Ops demands more variance; Killer Instinct demands more reads.

Execution · Competitive PvP
Cold War-era shooter pairing twitch multiplayer with the cult Zombies mode.

Execution · Competitive PvP
A combo-centric fighting game built around the breaker guessing game.
Call of Duty: Black Ops leans Execution (Competitive PvP); Killer Instinct leans Execution (Competitive PvP).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Call of Duty: Black Ops
Gunfights hinge on aim and reaction speed
Killer Instinct
Long combo execution and breaker timing
Killer Instinct leans clearly harder on reads.
Call of Duty: Black Ops
Reading human opponents' positioning and timing
Killer Instinct
Combo-breaker guessing is a live mind game
Call of Duty: Black Ops leans clearly harder on variance.
Call of Duty: Black Ops
Spawns, care packages, zombies RNG
Killer Instinct
Full information fighter
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Call of Duty: Black Ops
Class builds, map control, zombies routing
Killer Instinct
Frame data and matchup study
A matching score does not mean a matching test. Two games can both sit at 10 on execution and demand completely different things of you - which is why the reasoning above matters more than the number beside it.