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

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

Execution · Practiced Mastery
Co-op FPS about four masked crooks holding out through scripted heist assaults.
Grand Theft Auto V leans Execution (Practiced Mastery); Payday: The Heist leans Execution (Practiced Mastery).
Payday: The Heist leans a little harder on execution, though the gap is small.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Payday: The Heist
Holding off assault waves takes solid gunplay
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Payday: The Heist
Police waves are scripted spawns
Payday: The Heist leans clearly harder on variance.
Grand Theft Auto V
Some mission and lobby randomness.
Payday: The Heist
Drill failures and objective locations shuffle
Payday: The Heist leans a little harder on optimization, though the gap is small.
Grand Theft Auto V
Heists, money loops, builds learnable.
Payday: The Heist
Crew positioning and objective sequencing 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.