head to head
Neither is simply harder - they are hard in different places. Call of Juarez: Bound In Blood demands more execution; Grand Theft Auto: San Andreas demands more optimization.

Execution · Practiced Mastery
A linear western shooter testing aim and learned encounter pacing through scripted set pieces.

Optimization · Practiced Mastery
A sprawling open-world crime epic with RPG-style character stats across a whole state.
Call of Juarez: Bound In Blood leans Execution (Practiced Mastery); Grand Theft Auto: San Andreas leans Optimization (Practiced Mastery).
Call of Juarez: Bound In Blood leans clearly harder on execution.
Call of Juarez: Bound In Blood
Aimbot would trivialise the gunfights and showdown duels
Grand Theft Auto: San Andreas
Flying school and drive-bys need decent hands
Call of Juarez: Bound In Blood leans a little harder on reads, though the gap is small.
Call of Juarez: Bound In Blood
Scripted campaign enemies; thin multiplayer adds little prediction
Grand Theft Auto: San Andreas
Gang AI is scripted, never adaptive
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Call of Juarez: Bound In Blood
Encounter layouts are unknown on first run, then fixed
Grand Theft Auto: San Andreas
Territory wars and traffic add light randomness
Grand Theft Auto: San Andreas leans clearly harder on optimization.
Call of Juarez: Bound In Blood
Cover routes and concentration-mode usage are learnable patterns
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.