head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto: San Andreas demands more optimization; Psychonauts demands more execution.

Optimization · Practiced Mastery
A sprawling open-world crime epic with RPG-style character stats across a whole state.

Execution · Practiced Mastery
Moderate platforming execution through inventive but fixed mental levels.
Grand Theft Auto: San Andreas leans Optimization (Practiced Mastery); Psychonauts leans Execution (Practiced Mastery).
Psychonauts leans clearly harder on execution.
Grand Theft Auto: San Andreas
Flying school and drive-bys need decent hands
Psychonauts
Platforming and psychic combat need moderate precision
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Grand Theft Auto: San Andreas
Gang AI is scripted, never adaptive
Psychonauts
Mental-world enemies follow fixed behaviours
Grand Theft Auto: San Andreas leans a little harder on variance, though the gap is small.
Grand Theft Auto: San Andreas
Territory wars and traffic add light randomness
Psychonauts
Level layouts are fixed and quickly known
Grand Theft Auto: San Andreas leans clearly harder on optimization.
Grand Theft Auto: San Andreas
Stats, gym, weapon skill, and routes reward systematic play
Psychonauts
Collectible routing and power upgrades add light planning
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.