head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; Sonic Generations demands more execution.

Execution · Practiced Mastery
Sandbox crime playground spanning shooting, driving, and a persistent online economy.

Execution · Practiced Mastery
Executing memorised high-speed routes through deterministic platforming levels.
Grand Theft Auto V leans Execution (Practiced Mastery); Sonic Generations leans Execution (Practiced Mastery).
Sonic Generations leans clearly harder on execution.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Sonic Generations
Holding speed through precise jumps and boosts is the test
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Sonic Generations
No opponent; bosses are fixed patterns
Grand Theft Auto V leans clearly harder on variance.
Grand Theft Auto V
Some mission and lobby randomness.
Sonic Generations
Levels are fully deterministic
Sonic Generations leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
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.