head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; Mario & Luigi: Superstar Saga demands more optimization.

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

Optimization · Practiced Mastery
Comedy RPG where every turn-based attack hides an action-timing minigame.
Grand Theft Auto V leans Execution (Practiced Mastery); Mario & Luigi: Superstar Saga leans Optimization (Practiced Mastery).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Mario & Luigi: Superstar Saga
Timed hits and dodge jumps every turn
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Mario & Luigi: Superstar Saga
Bosses telegraph fixed patterns
Grand Theft Auto V leans a little harder on variance, though the gap is small.
Grand Theft Auto V
Some mission and lobby randomness.
Mario & Luigi: Superstar Saga
Encounters are authored
Mario & Luigi: Superstar Saga leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
Mario & Luigi: Superstar Saga
Bros. Attack mastery and stat builds
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.