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
A fast-paced military shooter where twitch aim against other players is the core test.

Execution · Pure Mechanics
Fast arena-style shooter mixing a scripted Strogg campaign with twitch multiplayer.
Call of Duty: Modern Warfare 3 leans Execution (Pure Mechanics); Quake 4 leans Execution (Pure Mechanics).
Call of Duty: Modern Warfare 3 leans a little harder on execution, though the gap is small.
Call of Duty: Modern Warfare 3
aim and reflexes decide most engagements
Quake 4
Aimbot plus strafe-jump inputs dominate both campaign and multiplayer
Call of Duty: Modern Warfare 3 leans a little harder on reads, though the gap is small.
Call of Duty: Modern Warfare 3
predicting human opponent positioning and habits
Quake 4
Multiplayer duels reward predicting human positioning and timing
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 3
spawn placement and loadout drops vary
Quake 4
Spawn and item timing add mild uncertainty
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 3
loadout tuning and map knowledge help
Quake 4
Item control routes and weapon picks 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.