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

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

Execution · Practiced Mastery
Executing memorised high-speed routes through deterministic platforming levels.
Grand Theft Auto: San Andreas leans Optimization (Practiced Mastery); Sonic Generations leans Execution (Practiced Mastery).
Sonic Generations leans clearly harder on execution.
Grand Theft Auto: San Andreas
Flying school and drive-bys need decent hands
Sonic Generations
Holding speed through precise jumps and boosts is the test
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
Sonic Generations
No opponent; bosses are fixed patterns
Grand Theft Auto: San Andreas leans clearly harder on variance.
Grand Theft Auto: San Andreas
Territory wars and traffic add light randomness
Sonic Generations
Levels are fully deterministic
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Grand Theft Auto: San Andreas
Stats, gym, weapon skill, and routes reward systematic play
Sonic Generations
S-ranks come from memorising optimal routes and shortcuts
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.