head to head
Neither is simply harder - they are hard in different places. Portal demands more execution; The Darkside Detective demands more variance.

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

Optimization · Pure Optimization
Bite-sized item-combination puzzles wrapped in comedy occult cases.
Portal leans Optimization (Pure Optimization); The Darkside Detective leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Portal
Portal placement timing modest.
The Darkside Detective
Pointing and clicking has no physical bar
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 Darkside Detective
Cases are authored scripts, not adapting minds
The Darkside Detective leans clearly harder on variance.
Portal
Deterministic chambers; no randomness.
The Darkside Detective
Solutions are hidden information until items are found
Portal leans clearly harder on optimization.
Portal
Pure puzzle-solving optimization.
The Darkside Detective
Item-combination puzzles each have one fixed answer
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.