head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; Mega Man X6 demands more execution.

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

Execution · Practiced Mastery
Punishing platforming execution over learnable boss orders and part builds.
Grand Theft Auto V leans Execution (Practiced Mastery); Mega Man X6 leans Execution (Practiced Mastery).
Mega Man X6 leans clearly harder on execution.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Mega Man X6
Cruel platforming and boss windows demand frame-tight inputs
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Mega Man X6
Bosses are fixed pattern sets
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.
Mega Man X6
The Nightmare System shuffles level hazards between runs
Mega Man X6 leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
Mega Man X6
Weapon orders, armor parts and route knowledge are essential
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.