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

Execution · Practiced Mastery
Landmark scripted FPS following Gordon Freeman through the Black Mesa disaster.
Bulletstorm leans Execution (Practiced Mastery); Half-Life leans Execution (Practiced Mastery).
Bulletstorm leans a little harder on execution, though the gap is small.
Bulletstorm
Leash, kick, and shot timing drive skillshot chains
Half-Life
Combat aim and infamous platforming sections
Bulletstorm leans a little harder on reads, though the gap is small.
Bulletstorm
Enemies are scripted fodder for your combos
Half-Life
Marines flank by script, not by modelling you
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
Half-Life
Set pieces surprise only on first run
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Bulletstorm
Maximising skillshot score per arena is a solvable puzzle
Half-Life
Resource routing and encounter knowledge for speedruns
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.