head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; Metroid Prime 3: Corruption demands more optimization.

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

Optimization · Practiced Mastery
A first-person Metroid adventure spanning planets, steered with the Wii's motion-aimed cannon.
Grand Theft Auto V leans Execution (Practiced Mastery); Metroid Prime 3: Corruption leans Optimization (Practiced Mastery).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Metroid Prime 3: Corruption
Pointer aiming and Hypermode management in fights.
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Metroid Prime 3: Corruption
Scripted bosses and pirates.
Grand Theft Auto V leans a little harder on variance, though the gap is small.
Grand Theft Auto V
Some mission and lobby randomness.
Metroid Prime 3: Corruption
Deterministic worlds.
Metroid Prime 3: Corruption leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
Metroid Prime 3: Corruption
Scanning, sequencing, and multi-planet routing drive progress.
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.