head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; Metroid Prime 2: Echoes demands more optimization.

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

Optimization · Practiced Mastery
Darker Prime sequel built on mirrored light and dark world traversal.
Grand Theft Auto V leans Execution (Practiced Mastery); Metroid Prime 2: Echoes leans Optimization (Practiced Mastery).
Grand Theft Auto V leans a little harder on execution, though the gap is small.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Metroid Prime 2: Echoes
lock-on combat, harsher but still forgiving aim
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Metroid Prime 2: Echoes
Ing follow scripted patterns
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Grand Theft Auto V
Some mission and lobby randomness.
Metroid Prime 2: Echoes
dark-world hazards surprise until learned
Metroid Prime 2: Echoes leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
Metroid Prime 2: Echoes
dual-world routing and key hunts reward planning
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.