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

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

Execution · Practiced Mastery
A test of 3D movement control with a deep routing layer underneath.
Grand Theft Auto V leans Execution (Practiced Mastery); Super Mario 64 leans Execution (Practiced Mastery).
Super Mario 64 leans clearly harder on execution.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Super Mario 64
Perfect analog inputs trivialise precise jumps, dives and wall-kicks
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Super Mario 64
Solo platformer - no adaptive opponent anywhere
Grand Theft Auto V leans a little harder on variance, though the gap is small.
Grand Theft Auto V
Some mission and lobby randomness.
Super Mario 64
Star locations and level layouts are fixed once seen
Super Mario 64 leans a little harder on optimization, though the gap is small.
Grand Theft Auto V
Heists, money loops, builds learnable.
Super Mario 64
Star routing and movement tech form a deep solvable layer
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.