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

Execution · Practiced Mastery
Straightforward cover-shooting aim against fully scripted enemy waves.

Execution · Practiced Mastery
A grittier crime story where weighty driving and gunplay carry the experience.
Army of Two: The Devil's Cartel leans Execution (Practiced Mastery); Grand Theft Auto IV 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.
Army of Two: The Devil's Cartel
Cover shooting rewards decent aim, nothing more
Grand Theft Auto IV
Heavier driving and cover shooting reward control.
Grand Theft Auto IV leans a little harder on reads, though the gap is small.
Army of Two: The Devil's Cartel
Cartel enemies follow scripted cover AI
Grand Theft Auto IV
Scripted AI, no adaptive opponent modeling.
Grand Theft Auto IV leans a little harder on variance, though the gap is small.
Army of Two: The Devil's Cartel
Encounters replay identically
Grand Theft Auto IV
Minor mission variance.
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Army of Two: The Devil's Cartel
Weapon customization adds light loadout math
Grand Theft Auto IV
Mission routing and weapon picks 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.