head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; Metal Slug Advance demands more execution.

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

Execution · Practiced Mastery
A handheld run-and-gun testing dodge precision and spawn memorisation.
Grand Theft Auto V leans Execution (Practiced Mastery); Metal Slug Advance leans Execution (Practiced Mastery).
Metal Slug Advance leans clearly harder on execution.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Metal Slug Advance
Dodging dense fire with one life bar demands precision
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Metal Slug Advance
Enemy soldiers spawn on strict scripts
Grand Theft Auto V leans a little harder on variance, though the gap is small.
Grand Theft Auto V
Some mission and lobby randomness.
Metal Slug Advance
Card drops add light collection randomness
Metal Slug Advance leans a little harder on optimization, though the gap is small.
Grand Theft Auto V
Heists, money loops, builds learnable.
Metal Slug Advance
Memorising spawns and card abilities optimises every mission
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.