head to head
Neither is meaningfully harder - they pose close to the same test. The widest gap is reads, and it is a single point.

Execution · Practiced Mastery
A gentle licensed racer testing basic driving lines against scripted opponents.

Execution · Practiced Mastery
Sandbox crime playground spanning shooting, driving, and a persistent online economy.
Cars leans Execution (Practiced Mastery); Grand Theft Auto V leans Execution (Practiced Mastery).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Cars
Forgiving kart-style driving with modest racing line demands
Grand Theft Auto V
Driving and shooting demand decent inputs.
Grand Theft Auto V leans a little harder on reads, though the gap is small.
Cars
AI racers follow rubber-band scripts; split-screen rivals barely adapt
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Grand Theft Auto V leans a little harder on variance, though the gap is small.
Cars
Tracks and events are fixed once seen
Grand Theft Auto V
Some mission and lobby randomness.
Grand Theft Auto V leans a little harder on optimization, though the gap is small.
Cars
Learning racing lines and shortcuts gives most of the edge
Grand Theft Auto V
Heists, money loops, builds learnable.
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.