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

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

Execution · Practiced Mastery
Nintendo's branching rail shooter where score medals demand perfected routes.
Grand Theft Auto V leans Execution (Practiced Mastery); Star Fox 64 leans Execution (Practiced Mastery).
Star Fox 64 leans clearly harder on execution.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Star Fox 64
Rail-shooter aim, barrel rolls, and boost timing are everything
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Star Fox 64
Entirely scripted bosses and waves
Grand Theft Auto V leans a little harder on variance, though the gap is small.
Grand Theft Auto V
Some mission and lobby randomness.
Star Fox 64
Fully deterministic once routes are known
Star Fox 64 leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
Star Fox 64
Medal scores and route branches reward total memorization
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.