head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; Sanctum 2 demands more optimization.

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

Optimization · Practiced Mastery
Hybrid tower-defense testing maze optimization backed by first-person aim.
Grand Theft Auto V leans Execution (Practiced Mastery); Sanctum 2 leans Optimization (Practiced Mastery).
Sanctum 2 leans a little harder on execution, though the gap is small.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Sanctum 2
Weak-point shooting during waves carries real aim demands
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Sanctum 2
Alien waves path predictably toward the core
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Grand Theft Auto V
Some mission and lobby randomness.
Sanctum 2
Wave compositions vary mildly between maps
Sanctum 2 leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
Sanctum 2
Maze layouts and tower economics are the solvable heart
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.