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

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

Optimization · Pure Optimization
Grid-based tactical combat solved through positioning and builds.
Portal leans Optimization (Pure Optimization); South Park: The Fractured But Whole leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Portal
Portal placement timing modest.
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
No adversary modeling player.
South Park: The Fractured But Whole
Enemy turns follow fixed visible logic
South Park: The Fractured But Whole leans clearly harder on variance.
Portal
Deterministic chambers; no randomness.
South Park: The Fractured But Whole
Minor roll spread; encounters are otherwise fixed
Portal leans a little harder on optimization, though the gap is small.
Portal
Pure puzzle-solving optimization.
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.