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

Execution · Practiced Mastery
The trilogy capper adding a Batmobile to the familiar freeflow formula.

Optimization · Practiced Mastery
A sprawling open-world crime epic with RPG-style character stats across a whole state.
Batman: Arkham Knight leans Execution (Practiced Mastery); Grand Theft Auto: San Andreas leans Optimization (Practiced Mastery).
Batman: Arkham Knight leans a little harder on execution, though the gap is small.
Batman: Arkham Knight
Combat and Batmobile control need timing.
Grand Theft Auto: San Andreas
Flying school and drive-bys need decent hands
Batman: Arkham Knight leans a little harder on reads, though the gap is small.
Batman: Arkham Knight
Scripted enemy AI.
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: Arkham Knight
Deterministic encounters.
Grand Theft Auto: San Andreas
Territory wars and traffic add light randomness
Grand Theft Auto: San Andreas leans a little harder on optimization, though the gap is small.
Batman: Arkham Knight
Gadget chains and tank routes learnable.
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.