head to head
Neither is simply harder - they are hard in different places. Portal demands more execution; Return of the Obra Dinn demands more optimization.

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

Optimization · Pure Optimization
A pure deduction puzzle with a single solvable answer set.
Portal leans Optimization (Pure Optimization); Return of the Obra Dinn leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Portal
Portal placement timing modest.
Return of the Obra Dinn
No mechanical execution required
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.
Return of the Obra Dinn
Static scenes, no opponent
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Portal
Deterministic chambers; no randomness.
Return of the Obra Dinn
Fully deterministic puzzle
Return of the Obra Dinn leans clearly harder on optimization.
Portal
Pure puzzle-solving optimization.
Return of the Obra Dinn
Pure deductive logic on a fixed manifest
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.