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

Execution · Pure Mechanics
Routine shooter aim through scripted cartel firefights.

Execution · Pure Mechanics
A gory budget FPS testing twitch aim through linear scripted firefights.
Call of Juarez: The Cartel leans Execution (Pure Mechanics); Soldier of Fortune: Payback leans Execution (Pure Mechanics).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Call of Juarez: The Cartel
Standard FPS gunfights run on aim alone
Soldier of Fortune: Payback
Twitch gunfights against aggressive AI are the core
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Call of Juarez: The Cartel
Co-op secret objectives add mild teammate suspicion
Soldier of Fortune: Payback
Multiplayer existed but campaign AI never adapts
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Call of Juarez: The Cartel
Enemy waves spawn identically each run
Soldier of Fortune: Payback
Linear levels hold few surprises
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Call of Juarez: The Cartel
Weapon levelling is a thin progression layer
Soldier of Fortune: Payback
Loadout picks add only a thin layer
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.