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

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

Optimization · Pure Optimization
Telltale's episodic comedy adventure sending Guybrush Threepwood after a pox plaguing the Caribbean.
Portal leans Optimization (Pure Optimization); Tales of Monkey Island leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Portal
Portal placement timing modest.
Tales of Monkey Island
Click-driven; no physical demands.
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.
Tales of Monkey Island
LeChuck and pirates are pure script.
Tales of Monkey Island leans clearly harder on variance.
Portal
Deterministic chambers; no randomness.
Tales of Monkey Island
Modern design telegraphs most puzzle information.
Portal leans clearly harder on optimization.
Portal
Pure puzzle-solving optimization.
Tales of Monkey Island
Episodic inventory puzzles each have one intended solution.
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.