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
An arcade FPS that scores you for killing enemies in the most creative ways possible.

Execution · Practiced Mastery
A sci-fi shooter where aim and weapon-switching against scripted alien enemies is the core test.
Bulletstorm leans Execution (Practiced Mastery); Halo: Combat Evolved Anniversary 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.
Bulletstorm
Leash, kick, and shot timing drive skillshot chains
Halo: Combat Evolved Anniversary
aim and grenade timing carry combat
Halo: Combat Evolved Anniversary leans a little harder on reads, though the gap is small.
Bulletstorm
Enemies are scripted fodder for your combos
Halo: Combat Evolved Anniversary
covenant AI is scripted, not adaptive
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Bulletstorm
Arenas and spawns are fixed and learnable
Halo: Combat Evolved Anniversary
enemy spawns and drops vary
Bulletstorm leans a little harder on optimization, though the gap is small.
Bulletstorm
Maximising skillshot score per arena is a solvable puzzle
Halo: Combat Evolved Anniversary
weapon sandbox and routes are solvable
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.