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

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

Execution · Practiced Mastery
Sonic alternates breakneck daytime boost stages with brawling night levels as the Werehog.
Grand Theft Auto V leans Execution (Practiced Mastery); Sonic Unleashed leans Execution (Practiced Mastery).
Sonic Unleashed leans clearly harder on execution.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Sonic Unleashed
Daytime boost stages demand reaction-speed precision
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Sonic Unleashed
Scripted enemies in both forms
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 Unleashed
First-run stage hazards arrive fast and unseen
Sonic Unleashed leans a little harder on optimization, though the gap is small.
Grand Theft Auto V
Heists, money loops, builds learnable.
Sonic Unleashed
S-rank routes and Werehog combat upgrades are solvable
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.