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

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

Execution · Practiced Mastery
Precise stunt driving over loops and jumps plus route optimisation per track.
Grand Theft Auto V leans Execution (Practiced Mastery); Stunts leans Execution (Practiced Mastery).
Stunts leans clearly harder on execution.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Stunts
Threading loops and jumps without crashing demands precise driving
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Stunts
AI opponents follow fixed lines
Grand Theft Auto V leans a little harder on variance, though the gap is small.
Grand Theft Auto V
Some mission and lobby randomness.
Stunts
Tracks are known; physics quirks barely random
Stunts leans a little harder on optimization, though the gap is small.
Grand Theft Auto V
Heists, money loops, builds learnable.
Stunts
Optimal lines and car choice per track are solvable
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.