head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; Luxor 3 demands more execution.

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

Execution · Practiced Mastery
A marble-shooter testing aim and timing to match advancing colored spheres.
Grand Theft Auto V leans Execution (Practiced Mastery); Luxor 3 leans Execution (Practiced Mastery).
Luxor 3 leans clearly harder on execution.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Luxor 3
Aiming and timing shots at moving spheres is the core
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Luxor 3
Solo - no opponent to predict
Luxor 3 leans clearly harder on variance.
Grand Theft Auto V
Some mission and lobby randomness.
Luxor 3
Sphere color sequences add randomness
Luxor 3 leans a little harder on optimization, though the gap is small.
Grand Theft Auto V
Heists, money loops, builds learnable.
Luxor 3
Power-up usage and chain planning improve scores
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.