head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; Sonic the Hedgehog 3 demands more execution.

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

Execution · Practiced Mastery
Momentum-driven execution over fixed levels rewarding memorised routes.
Grand Theft Auto V leans Execution (Practiced Mastery); Sonic the Hedgehog 3 leans Execution (Practiced Mastery).
Sonic the Hedgehog 3 leans clearly harder on execution.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Sonic the Hedgehog 3
Holding speed through fixed hazards demands sharp reactions
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Sonic the Hedgehog 3
Badniks and bosses repeat identical scripts
Grand Theft Auto V leans a little harder on variance, though the gap is small.
Grand Theft Auto V
Some mission and lobby randomness.
Sonic the Hedgehog 3
Levels never change between runs
Sonic the Hedgehog 3 leans a little harder on optimization, though the gap is small.
Grand Theft Auto V
Heists, money loops, builds learnable.
Sonic the Hedgehog 3
Route choice and emerald collection reward memorised 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.