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
A grittier crime story where weighty driving and gunplay carry the experience.

Execution · Practiced Mastery
Linear postwar crime story driven by cover shootouts and atmosphere.
Grand Theft Auto IV leans Execution (Practiced Mastery); Mafia II leans Execution (Practiced Mastery).
Grand Theft Auto IV leans a little harder on execution, though the gap is small.
Grand Theft Auto IV
Heavier driving and cover shooting reward control.
Mafia II
Cover shooting and fistfight timing
Grand Theft Auto IV leans a little harder on reads, though the gap is small.
Grand Theft Auto IV
Scripted AI, no adaptive opponent modeling.
Mafia II
Enemies are scripted
Grand Theft Auto IV leans a little harder on variance, though the gap is small.
Grand Theft Auto IV
Minor mission variance.
Mafia II
Story missions are fixed
Mafia II leans a little harder on optimization, though the gap is small.
Grand Theft Auto IV
Mission routing and weapon picks learnable.
Mafia II
Modest; learning encounter layouts
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.