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

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

Execution · Practiced Mastery
Twitch dodging and shooting layered over memorised arcade stage routes.
Grand Theft Auto V leans Execution (Practiced Mastery); Metal Slug leans Execution (Practiced Mastery).
Metal Slug leans clearly harder on execution.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Metal Slug
Dodging dense bullet patterns and precise shots carry every credit
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Metal Slug
Enemies follow fixed arcade scripts, never adapt
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
Spawns are nearly deterministic once a stage is learned
Metal Slug leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
Metal Slug
One-credit clears come from memorising routes and spawn patterns
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.