head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; Pilotwings 64 demands more execution.

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

Execution · Practiced Mastery
Flight school score-attack where smooth control of quirky aircraft earns medals.
Grand Theft Auto V leans Execution (Practiced Mastery); Pilotwings 64 leans Execution (Practiced Mastery).
Pilotwings 64 leans clearly harder on execution.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Pilotwings 64
Precise stick control of gyrocopter and jetpack
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Pilotwings 64
No opposition, only score targets
Grand Theft Auto V leans a little harder on variance, though the gap is small.
Grand Theft Auto V
Some mission and lobby randomness.
Pilotwings 64
Courses fully fixed
Pilotwings 64 leans a little harder on optimization, though the gap is small.
Grand Theft Auto V
Heists, money loops, builds learnable.
Pilotwings 64
Optimal lines and landings reward study
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.