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

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

Optimization · Pure Optimization
Comedic point-and-click adventure of Guybrush Threepwood, hand-drawn at its peak.
Portal leans Optimization (Pure Optimization); The Curse of Monkey Island leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Portal
Portal placement timing modest.
The Curse of Monkey Island
pointing and clicking only
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 Curse of Monkey Island
no adaptive opponent
The Curse of Monkey Island leans clearly harder on variance.
Portal
Deterministic chambers; no randomness.
The Curse of Monkey Island
puzzle solutions unknown on first play, then fixed
Portal leans a little harder on optimization, though the gap is small.
Portal
Pure puzzle-solving optimization.
The Curse of Monkey Island
inventory logic puzzles are fully solvable
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.