head to head
Neither is meaningfully harder - they pose close to the same test. The widest gap is variance, and it is a single point.

Optimization · Practiced Mastery
Darker Prime sequel built on mirrored light and dark world traversal.

Optimization · Pure Optimization
Colour-painting terrain puzzles that resolve to finding the intended solution.
Metroid Prime 2: Echoes leans Optimization (Practiced Mastery); Nihilumbra leans Optimization (Pure Optimization).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Metroid Prime 2: Echoes
lock-on combat, harsher but still forgiving aim
Nihilumbra
Some platforming timing, but forgiving throughout
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Metroid Prime 2: Echoes
Ing follow scripted patterns
Nihilumbra
Enemies cannot be fought or read, only avoided
Metroid Prime 2: Echoes leans a little harder on variance, though the gap is small.
Metroid Prime 2: Echoes
dark-world hazards surprise until learned
Nihilumbra
Hand-designed puzzles with nothing random
Nihilumbra leans a little harder on optimization, though the gap is small.
Metroid Prime 2: Echoes
dual-world routing and key hunts reward planning
Nihilumbra
Choosing which colour solves each terrain puzzle is the game
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.