head to head
Neither is simply harder - they are hard in different places. Grid 2 demands more optimization; Rocket League demands more reads.

Execution · Practiced Mastery
Cornering execution over memorisable circuits, with light prediction in wheel-to-wheel multiplayer.

Execution · Pure Mechanics
Mechanical car-soccer where execution is the ceiling.
Grid 2 leans Execution (Practiced Mastery); Rocket League leans Execution (Pure Mechanics).
Rocket League leans clearly harder on execution.
Grid 2
Car control through corners is the entire moment-to-moment test
Rocket League
Aerials, flicks, dribbles, pure mechanical ceiling
Rocket League leans clearly harder on reads.
Grid 2
Online wheel-to-wheel adds light positional prediction
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.
Grid 2
Tracks are fixed and fully visible
Rocket League
Almost deterministic physics
Grid 2 leans clearly harder on optimization.
Grid 2
Racing lines, braking points and car selection are studyable
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.