head to head
Neither is harder. They score identically on all four axes, so the difference between them is in kind, not degree.

Execution · Pure Mechanics
The 70s-cinema getaway driving game with famously demanding physics.

Execution · Pure Mechanics
Zero-gravity physics game about wrestling floppy cosmonaut limbs through space-station chores.
Driver leans Execution (Pure Mechanics); Heavenly Bodies leans Execution (Pure Mechanics).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Driver
Handbrake control and pursuit driving by feel
Heavenly Bodies
Deliberately awkward limb-by-limb zero-g control
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Driver
Police AI is mappable
Heavenly Bodies
No opposition
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Driver
Traffic adds mild unpredictability
Heavenly Bodies
Fixed scenario layouts
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Driver
City routes and mission tactics learnable
Heavenly Bodies
Task order and clever anchor points
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.