head to head
Neither is meaningfully harder - they pose close to the same test. The widest gap is execution, and it is a single point.

Execution · Pure Mechanics
Treyarch's future-tech Call of Duty that anchored competitive console FPS.

Execution · Competitive PvP
Tactical gunfights testing aim, entry planning and prediction of human opponents online.
Call of Duty: Black Ops II leans Execution (Pure Mechanics); Tom Clancy's Rainbow Six: Vegas leans Execution (Competitive PvP).
Call of Duty: Black Ops II leans a little harder on execution, though the gap is small.
Call of Duty: Black Ops II
Snap aim and movement speed decide split-second gunfights
Tom Clancy's Rainbow Six: Vegas
Aim and snap-to-cover gunplay punish slow hands
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Call of Duty: Black Ops II
Human lobbies adapt rotations; pros mind-game spawns
Tom Clancy's Rainbow Six: Vegas
Multiplayer angles and pushes demand reading human opponents
Call of Duty: Black Ops II leans a little harder on variance, though the gap is small.
Call of Duty: Black Ops II
Spawn flips and scorestreak chaos inject randomness
Tom Clancy's Rainbow Six: Vegas
Terrorist hunt spawns vary modestly between runs
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 II
Pick-10 classes and streak economies reward optimization
Tom Clancy's Rainbow Six: Vegas
Room-entry planning and map knowledge are studyable
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.