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

Optimization · Pure Optimization
Colour-painting terrain puzzles that resolve to finding the intended solution.

Optimization · Practiced Mastery
A puzzle-leaning platformer testing perspective-flip problem solving more than reflexes.
Nihilumbra leans Optimization (Pure Optimization); Super Paper Mario leans Optimization (Practiced Mastery).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Nihilumbra
Some platforming timing, but forgiving throughout
Super Paper Mario
Light platforming demands, well below mainline Mario
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Nihilumbra
Enemies cannot be fought or read, only avoided
Super Paper Mario
Enemies follow simple fixed patterns, no adaptation
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Nihilumbra
Hand-designed puzzles with nothing random
Super Paper Mario
Dimension-flip secrets are scripted-but-unknown until found
Nihilumbra leans a little harder on optimization, though the gap is small.
Nihilumbra
Choosing which colour solves each terrain puzzle is the game
Super Paper Mario
Solving levels via the 2D/3D flip mechanic is the core test
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.