head to head
Neither is simply harder - they are hard in different places. Eriksholm: The Stolen Dream demands more variance; Portal 2 demands more optimization.

Optimization · Pure Optimization
Narrative stealth puzzles solved by planning routes through scripted patrols.

Optimization · Pure Optimization
A first-person puzzle game where seeing the solution is the whole skill.
Eriksholm: The Stolen Dream leans Optimization (Pure Optimization); Portal 2 leans Optimization (Pure Optimization).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Eriksholm: The Stolen Dream
Timing patrol gaps needs only modest dexterity
Portal 2
Portal placement requires some precision but timing is lenient.
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Eriksholm: The Stolen Dream
Guards run fixed vision cones and routes
Portal 2
No opponent; puzzles are static.
Eriksholm: The Stolen Dream leans clearly harder on variance.
Eriksholm: The Stolen Dream
Each scene is fully readable before acting
Portal 2
Puzzles are deterministic with no randomness.
Portal 2 leans clearly harder on optimization.
Eriksholm: The Stolen Dream
Solving multi-character stealth routes is the whole game
Portal 2
Solving the puzzle's logic IS the entire game.
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.