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

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

Execution · Practiced Mastery
Arcade boss-rush testing pattern execution and weakness-order knowledge.
Grand Theft Auto V leans Execution (Practiced Mastery); Mega Man: The Power Battle leans Execution (Practiced Mastery).
Mega Man: The Power Battle leans clearly harder on execution.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Mega Man: The Power Battle
Boss pattern dodging at arcade pace tests the hands
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Mega Man: The Power Battle
Robot Masters cycle fixed movesets
Grand Theft Auto V leans a little harder on variance, though the gap is small.
Grand Theft Auto V
Some mission and lobby randomness.
Mega Man: The Power Battle
Boss selection order barely changes the run
Mega Man: The Power Battle leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
Mega Man: The Power Battle
Weapon-weakness ordering is a solvable puzzle
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.