head to head
Neither is simply harder - they are hard in different places. 3D Pinball: Space Cadet demands more execution; Grand Theft Auto: San Andreas demands more optimization.

Execution · Practiced Mastery
Flipper timing against bouncing physics, with table missions rewarding memorised routes.

Optimization · Practiced Mastery
A sprawling open-world crime epic with RPG-style character stats across a whole state.
3D Pinball: Space Cadet leans Execution (Practiced Mastery); Grand Theft Auto: San Andreas leans Optimization (Practiced Mastery).
3D Pinball: Space Cadet leans clearly harder on execution.
3D Pinball: Space Cadet
Flipper timing and nudge control are the score ceiling
Grand Theft Auto: San Andreas
Flying school and drive-bys need decent hands
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
3D Pinball: Space Cadet
A table cannot adapt to you
Grand Theft Auto: San Andreas
Gang AI is scripted, never adaptive
3D Pinball: Space Cadet leans a little harder on variance, though the gap is small.
3D Pinball: Space Cadet
Ball physics bounce beyond full control
Grand Theft Auto: San Andreas
Territory wars and traffic add light randomness
Grand Theft Auto: San Andreas leans clearly harder on optimization.
3D Pinball: Space Cadet
Mission sequences and table knowledge multiply scores
Grand Theft Auto: San Andreas
Stats, gym, weapon skill, and routes reward systematic play
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.