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

Execution · Practiced Mastery
Wrestling awkward scripted brawling controls through fixed, memorisable stages.

Optimization · Practiced Mastery
A sprawling open-world crime epic with RPG-style character stats across a whole state.
Batman Forever leans Execution (Practiced Mastery); Grand Theft Auto: San Andreas leans Optimization (Practiced Mastery).
Batman Forever leans clearly harder on execution.
Batman Forever
Clunky fighting-game inputs gate basic brawling moves
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.
Batman Forever
Enemies run fixed scripts with no adaptation
Grand Theft Auto: San Andreas
Gang AI is scripted, never adaptive
Grand Theft Auto: San Andreas leans a little harder on variance, though the gap is small.
Batman Forever
Stages and spawns are identical every playthrough
Grand Theft Auto: San Andreas
Territory wars and traffic add light randomness
Grand Theft Auto: San Andreas leans clearly harder on optimization.
Batman Forever
Learning the awkward training-required moves and level routes
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.