head to head
Neither is simply harder - they are hard in different places. Portal demands more execution; The Secret of Monkey Island demands more variance.

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

Optimization · Pure Optimization
A fixed-solution puzzle adventure where finding the right inventory combination unlocks every obstacle.
Portal leans Optimization (Pure Optimization); The Secret of Monkey Island leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Portal
Portal placement timing modest.
The Secret of Monkey Island
Clicking inventory items demands no twitch skill whatsoever
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 Secret of Monkey Island
No adaptive opponent; all puzzles are fixed and authored
The Secret of Monkey Island leans clearly harder on variance.
Portal
Deterministic chambers; no randomness.
The Secret of Monkey Island
Some hidden items require exploration but solutions are invariant
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Portal
Pure puzzle-solving optimization.
The Secret of Monkey Island
Correct item combinations and dialogue chains fully determine progress
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.