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

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

Execution · Practiced Mastery
Arcade-style western shooter built around stage gunfights and showdown duels.
Grand Theft Auto: San Andreas leans Optimization (Practiced Mastery); Red Dead Revolver leans Execution (Practiced Mastery).
Red Dead Revolver leans clearly harder on execution.
Grand Theft Auto: San Andreas
Flying school and drive-bys need decent hands
Red Dead Revolver
Arcade gunplay and duel timing dominate
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
Red Dead Revolver
Enemies are scripted shooting-gallery foes
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Grand Theft Auto: San Andreas
Territory wars and traffic add light randomness
Red Dead Revolver
Minor spawn variation
Grand Theft Auto: San Andreas leans clearly harder on optimization.
Grand Theft Auto: San Andreas
Stats, gym, weapon skill, and routes reward systematic play
Red Dead Revolver
Level layouts and duel rhythms are learnable
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.