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

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

Optimization · Practiced Mastery
Methodically mapping a hostile station where knowledge and routing beat reflexes.
Grand Theft Auto V leans Execution (Practiced Mastery); System Shock 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.
System Shock
Combat and cyberspace ask competent but dated reflexes
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
System Shock
SHODAN's threats are scripted, not an adapting mind
System Shock leans a little harder on variance, though the gap is small.
Grand Theft Auto V
Some mission and lobby randomness.
System Shock
Citadel's layout and logs are hidden until uncovered
System Shock leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
System Shock
Inventory, routing and station knowledge solve the game
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.