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

Optimization · Pure Optimization
A comedic point-and-click adventure of pirate puzzles with fixed solutions.

Optimization · Pure Optimization
The portal-gun puzzle game whose solutions are everything.
Escape from Monkey Island leans Optimization (Pure Optimization); Portal leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Escape from Monkey Island
Pointing and walking only
Portal
Portal placement timing modest.
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Escape from Monkey Island
No opponent
Portal
No adversary modeling player.
Escape from Monkey Island leans clearly harder on variance.
Escape from Monkey Island
Puzzle solutions hidden until discovered
Portal
Deterministic chambers; no randomness.
Portal leans clearly harder on optimization.
Escape from Monkey Island
Once known, every puzzle has one fixed answer
Portal
Pure puzzle-solving optimization.
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.