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

Execution · Practiced Mastery
Weighty corridor shooting through scripted encounters with light loadout choice.

Execution · Practiced Mastery
Third-person action where Captain Titus chainswords through ork hordes without a cover system.
Killzone leans Execution (Practiced Mastery); Warhammer 40,000: Space Marine leans Execution (Practiced Mastery).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Killzone
Heavy gunplay still rewards aim and positioning
Warhammer 40,000: Space Marine
Melee-shooting flow and execution timing carry combat
Killzone leans a little harder on reads, though the gap is small.
Killzone
Campaign Helghast follow scripts; servers are long dead
Warhammer 40,000: Space Marine
Ork hordes follow scripted aggression
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Killzone
Encounters are fixed and replayable
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.
Killzone
Character choice and route knowledge solve each level
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.