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

Execution · Pure Mechanics
Landmark FPS built on physics puzzles and tight gunplay.
Army of Two: The Devil's Cartel leans Execution (Practiced Mastery); Half-Life 2 leans Execution (Pure Mechanics).
Half-Life 2 leans a little harder on execution, though the gap is small.
Army of Two: The Devil's Cartel
Cover shooting rewards decent aim, nothing more
Half-Life 2
Gravity gun and shooting demand precision.
Half-Life 2 leans a little harder on reads, though the gap is small.
Army of Two: The Devil's Cartel
Cartel enemies follow scripted cover AI
Half-Life 2
Combine AI scripted, non-adaptive.
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Army of Two: The Devil's Cartel
Encounters replay identically
Half-Life 2
Almost no randomness.
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
Half-Life 2
Linear progression with no builds.
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.