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

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

Execution · Practiced Mastery
Remake of the original Metroid with modern movement and a stealth epilogue.
Grand Theft Auto V leans Execution (Practiced Mastery); Metroid: Zero Mission leans Execution (Practiced Mastery).
Metroid: Zero Mission leans a little harder on execution, though the gap is small.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Metroid: Zero Mission
polished 2D movement plus a stealth finale
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Metroid: Zero Mission
enemies and pirates are scripted
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: Zero Mission
hidden passages unknown only once
Metroid: Zero Mission leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
Metroid: Zero Mission
sequence breaks and route optimization are celebrated
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.