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

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

Optimization · Pure Optimization
Lateral-thinking puzzle solving that converges to a walkthrough once solutions are known.
Portal leans Optimization (Pure Optimization); The Secret of Monkey Island: Special Edition leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Portal
Portal placement timing modest.
The Secret of Monkey Island: Special Edition
Pointing and clicking has no physical ceiling
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: Special Edition
Insult swordfighting is scripted dialogue lookup, not prediction
The Secret of Monkey Island: Special Edition leans clearly harder on variance.
Portal
Deterministic chambers; no randomness.
The Secret of Monkey Island: Special Edition
Puzzle solutions are hidden information until discovered
Portal leans clearly harder on optimization.
Portal
Pure puzzle-solving optimization.
The Secret of Monkey Island: Special Edition
Once solutions are known the game is fully solved
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.