head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; Payday 2 demands more variance.

Execution · Practiced Mastery
Sandbox crime playground spanning shooting, driving, and a persistent online economy.

Execution · Practiced Mastery
Loot-driven co-op heist shooter where builds and route knowledge make perfect crimes.
Grand Theft Auto V leans Execution (Practiced Mastery); Payday 2 leans Execution (Practiced Mastery).
Payday 2 leans a little harder on execution, though the gap is small.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Payday 2
Sustained gunfights and stealth execution under pressure
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Payday 2
Cops and guards follow spawn scripts
Payday 2 leans clearly harder on variance.
Grand Theft Auto V
Some mission and lobby randomness.
Payday 2
Random vault locations, guard patrols, and escrow rolls
Payday 2 leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
Payday 2
Skill builds and stealth route planning per heist
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.