head to head
Neither is meaningfully harder - they pose close to the same test. The widest gap is execution, and it is a single point.

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

Execution · Practiced Mastery
Third-person action where Captain Titus chainswords through ork hordes without a cover system.
Grand Theft Auto V leans Execution (Practiced Mastery); Warhammer 40,000: Space Marine leans Execution (Practiced Mastery).
Warhammer 40,000: Space Marine leans a little harder on execution, though the gap is small.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Warhammer 40,000: Space Marine
Melee-shooting flow and execution timing carry combat
Grand Theft Auto V leans a little harder on reads, though the gap is small.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Warhammer 40,000: Space Marine
Ork hordes follow scripted aggression
Grand Theft Auto V leans a little harder on variance, though the gap is small.
Grand Theft Auto V
Some mission and lobby randomness.
Warhammer 40,000: Space Marine
Encounters are fixed and predictable
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Grand Theft Auto V
Heists, money loops, builds learnable.
Warhammer 40,000: Space Marine
Weapon choice and fury management are simple to learn
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.