head to head
Neither is harder. They score identically on all four axes, so the difference between them is in kind, not degree.

Execution · Practiced Mastery
Landmark scripted FPS following Gordon Freeman through the Black Mesa disaster.

Execution · Practiced Mastery
Remake of the original Metroid with modern movement and a stealth epilogue.
Half-Life leans Execution (Practiced Mastery); Metroid: Zero Mission 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.
Half-Life
Combat aim and infamous platforming sections
Metroid: Zero Mission
polished 2D movement plus a stealth finale
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Half-Life
Marines flank by script, not by modelling you
Metroid: Zero Mission
enemies and pirates are scripted
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Half-Life
Set pieces surprise only on first run
Metroid: Zero Mission
hidden passages unknown only once
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Half-Life
Resource routing and encounter knowledge for speedruns
Metroid: Zero Mission
sequence breaks and route optimization are celebrated
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.