head to head
Neither is simply harder - they are hard in different places. Portal demands more execution; The Dark Eye: Memoria demands more variance.

Optimization · Pure Optimization
The portal-gun puzzle game whose solutions are everything.

Optimization · Pure Optimization
Logical item-and-spell puzzle chains inside a fixed two-protagonist fantasy story.
Portal leans Optimization (Pure Optimization); The Dark Eye: Memoria leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Portal
Portal placement timing modest.
The Dark Eye: Memoria
Clicking hotspots has no dexterity component
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Portal
No adversary modeling player.
The Dark Eye: Memoria
Story beats are authored, nothing adapts to you
The Dark Eye: Memoria leans clearly harder on variance.
Portal
Deterministic chambers; no randomness.
The Dark Eye: Memoria
Puzzle solutions are hidden until experimentation reveals them
Portal leans clearly harder on optimization.
Portal
Pure puzzle-solving optimization.
The Dark Eye: Memoria
Combining items and spells logically is the entire 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.