head to head
Neither is simply harder - they are hard in different places. Race Driver: Grid demands more optimization; Rocket League demands more reads.

Execution · Practiced Mastery
Aggressive circuit racing testing car control over memorisable tracks and braking points.

Execution · Pure Mechanics
Mechanical car-soccer where execution is the ceiling.
Race Driver: Grid leans Execution (Practiced Mastery); Rocket League leans Execution (Pure Mechanics).
Rocket League leans clearly harder on execution.
Race Driver: Grid
Aggressive wheel-to-wheel car control is the core demand
Rocket League
Aerials, flicks, dribbles, pure mechanical ceiling
Rocket League leans clearly harder on reads.
Race Driver: Grid
Online racing adds light opponent positioning awareness
Rocket League
Rotations and 50-50 reads against humans
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Race Driver: Grid
Damage outcomes add slight unpredictability to contact
Rocket League
Almost deterministic physics
Race Driver: Grid leans clearly harder on optimization.
Race Driver: Grid
Braking points, lines and flashback management are learnable
Rocket League
Training routines, rotation theory
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.