head to head
Neither is simply harder - they are hard in different places. Portal 2 demands more optimization; South Park: The Fractured But Whole demands more variance.

Optimization · Pure Optimization
A first-person puzzle game where seeing the solution is the whole skill.

Optimization · Pure Optimization
Grid-based tactical combat solved through positioning and builds.
Portal 2 leans Optimization (Pure Optimization); South Park: The Fractured But Whole leans Optimization (Pure Optimization).
Portal 2 leans a little harder on execution, though the gap is small.
Portal 2
Portal placement requires some precision but timing is lenient.
South Park: The Fractured But Whole
Timing minigames barely gate the grid combat
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Portal 2
No opponent; puzzles are static.
South Park: The Fractured But Whole
Enemy turns follow fixed visible logic
South Park: The Fractured But Whole leans clearly harder on variance.
Portal 2
Puzzles are deterministic with no randomness.
South Park: The Fractured But Whole
Minor roll spread; encounters are otherwise fixed
Portal 2 leans clearly harder on optimization.
Portal 2
Solving the puzzle's logic IS the entire game.
South Park: The Fractured But Whole
Grid positioning and artifact builds solve every fight
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.