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

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

Execution · Practiced Mastery
A blistering DS Sonic built around the boost gauge, introducing Blaze the Cat.
Grand Theft Auto V leans Execution (Practiced Mastery); Sonic Rush leans Execution (Practiced Mastery).
Sonic Rush leans clearly harder on execution.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Sonic Rush
Boost management and trick timing at high speed
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Sonic Rush
Scripted bosses only
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 Rush
Dual-screen layouts surprise on first runs
Sonic Rush leans a little harder on optimization, though the gap is small.
Grand Theft Auto V
Heists, money loops, builds learnable.
Sonic Rush
Rank grinding turns levels into solved boost routes
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.