head to head
Neither is harder. They score identically on all four axes, so the difference between them is in kind, not degree.

Execution · Practiced Mastery
Aim and spawn memorisation through a fully scripted cinematic campaign.

Execution · Practiced Mastery
A Serious Sam-style horde shooter testing strafing aim against scripted waves.
Call of Duty: Modern Warfare 2 Campaign Remastered leans Execution (Practiced Mastery); Will Rock leans Execution (Practiced Mastery).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Call of Duty: Modern Warfare 2 Campaign Remastered
Aim and reaction carry every scripted firefight
Will Rock
Circle-strafing huge enemy swarms is the whole test
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Call of Duty: Modern Warfare 2 Campaign Remastered
Enemies spawn and behave on fixed scripts
Will Rock
Monsters charge in fixed spawned waves
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Call of Duty: Modern Warfare 2 Campaign Remastered
Set-piece campaign with almost nothing random
Will Rock
Spawns are scripted and repeat identically
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Call of Duty: Modern Warfare 2 Campaign Remastered
Learning spawns and cover lines trivialises veteran difficulty
Will Rock
Weapon choice and arena positioning are learnable
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.