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

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

Optimization · Pure Optimization
A sarcastic British comedy adventure where a teenage wizard rescues an old sorcerer from an evil rival.
Portal leans Optimization (Pure Optimization); Simon the Sorcerer leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Portal
Portal placement timing modest.
Simon the Sorcerer
Verb-and-click; zero reflex requirement.
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.
Simon the Sorcerer
Fantasy parodies are static set dressing.
Simon the Sorcerer leans clearly harder on variance.
Portal
Deterministic chambers; no randomness.
Simon the Sorcerer
Some obscure item uses surprise first playthroughs.
Portal leans clearly harder on optimization.
Portal
Pure puzzle-solving optimization.
Simon the Sorcerer
Classic inventory puzzle chains with fixed solutions.
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.