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
An on-rails aim-and-memorise test through fixed FMV sequences.

Execution · Practiced Mastery
Third-person action where Captain Titus chainswords through ork hordes without a cover system.
Star Wars: Rebel Assault 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.
Star Wars: Rebel Assault
On-rails aiming and dodge timing is the entire challenge
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.
Star Wars: Rebel Assault
Every sequence is pre-rendered and fixed
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.
Star Wars: Rebel Assault
Nothing varies between attempts
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.
Star Wars: Rebel Assault
Memorising each rail sequence's threats carries completion
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.