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

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

Optimization · Pure Optimization
Piecing together a scripted murder conspiracy from hidden clues.
Portal leans Optimization (Pure Optimization); Post Mortem leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Portal
Portal placement timing modest.
Post Mortem
Point-and-click interface, zero dexterity
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.
Post Mortem
Suspects follow scripted dialogue trees
Post Mortem leans clearly harder on variance.
Portal
Deterministic chambers; no randomness.
Post Mortem
The conspiracy is hidden information revealed by investigation
Portal leans clearly harder on optimization.
Portal
Pure puzzle-solving optimization.
Post Mortem
Piecing fixed clues into the solution 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.