head to head
Neither is meaningfully harder - they pose close to the same test. The widest gap is reads, and it is a single point.

Optimization · Pure Optimization
A VATS-heavy wasteland RPG won by perk and build optimization.

Optimization · Pure Optimization
A gentle turn-based system easily solved by build and gear choices.
Fallout 3 leans Optimization (Pure Optimization); South Park: The Stick of Truth 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.
Fallout 3
VATS-leaning combat, low aim demand
South Park: The Stick of Truth
Timed attack and block prompts add light input demands
Fallout 3 leans a little harder on reads, though the gap is small.
Fallout 3
Scripted enemies
South Park: The Stick of Truth
Enemies follow fixed turn-based scripts
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Fallout 3
Loot rolls, SPECIAL checks
South Park: The Stick of Truth
Minor damage rolls; encounters are otherwise fixed
Fallout 3 leans a little harder on optimization, though the gap is small.
Fallout 3
Perk and build planning carries the run
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.