head to head
Neither is simply harder - they are hard in different places. Portal demands more optimization; South Park: The Stick of Truth demands more variance.

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

Optimization · Pure Optimization
A gentle turn-based system easily solved by build and gear choices.
Portal leans Optimization (Pure Optimization); South Park: The Stick of Truth leans Optimization (Pure Optimization).
Portal leans a little harder on execution, though the gap is small.
Portal
Portal placement timing modest.
South Park: The Stick of Truth
Timed attack and block prompts add light input 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.
South Park: The Stick of Truth
Enemies follow fixed turn-based scripts
South Park: The Stick of Truth leans clearly harder on variance.
Portal
Deterministic chambers; no randomness.
South Park: The Stick of Truth
Minor damage rolls; encounters are otherwise fixed
Portal leans clearly harder on optimization.
Portal
Pure puzzle-solving optimization.
South Park: The Stick of Truth
Build, gear and ability choices comfortably 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.