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 · Pure Mechanics
Wrestling motion-control aim and swordplay through fixed scripted shootouts.

Execution · Practiced Mastery
Third-person action where Captain Titus chainswords through ork hordes without a cover system.
Red Steel leans Execution (Pure Mechanics); 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.
Red Steel
Wiimote aiming and sword swings are the whole physical test
Warhammer 40,000: Space Marine
Melee-shooting flow and execution timing carry combat
Warhammer 40,000: Space Marine leans a little harder on reads, though the gap is small.
Red Steel
Enemies are scripted spawns, never adaptive
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.
Red Steel
Encounters are fixed once seen
Warhammer 40,000: Space Marine
Encounters are fixed and predictable
Warhammer 40,000: Space Marine leans a little harder on optimization, though the gap is small.
Red Steel
Light level memorisation and weapon choices
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.