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

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

Execution · Practiced Mastery
The 3DS remake of Metroid II built around a precisely timed melee counter.
Grand Theft Auto V leans Execution (Practiced Mastery); Metroid: Samus Returns leans Execution (Practiced Mastery).
Metroid: Samus Returns leans a little harder on execution, though the gap is small.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Metroid: Samus Returns
Melee-counter timing defines nearly every fight.
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Metroid: Samus Returns
Counters punish scripted lunges, not adaptation.
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: Samus Returns
Fixed map and encounters.
Metroid: Samus Returns leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
Metroid: Samus Returns
Aeion management and ability routing matter.
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.