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

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

Optimization · Practiced Mastery
The 1986 original that defined exploration-driven, secret-riddled platforming.
Grand Theft Auto V leans Execution (Practiced Mastery); Metroid 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
stiff NES jumps and tough enemy contact
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Metroid
creatures follow fixed movement
Metroid leans a little harder on variance, though the gap is small.
Grand Theft Auto V
Some mission and lobby randomness.
Metroid
unmarked secrets and bombable blocks hide the map
Metroid leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
Metroid
route and item knowledge transforms the game
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.