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

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

Optimization · Practiced Mastery
Community resource management under randomised zombie pressure, with light action execution.
Grand Theft Auto V leans Execution (Practiced Mastery); State of Decay leans Optimization (Practiced Mastery).
Grand Theft Auto V leans a little harder on execution, though the gap is small.
Grand Theft Auto V
Driving and shooting demand decent inputs.
State of Decay
Melee and driving are functional, not demanding
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
State of Decay
Zombies swarm by simple rules, never adapting
State of Decay leans clearly harder on variance.
Grand Theft Auto V
Some mission and lobby randomness.
State of Decay
Dynamic events, infestations and survivor traits roll randomly
State of Decay leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
State of Decay
Base building, resource runs and survivor management are the game
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.