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
Cornering execution over memorisable circuits, with light prediction in wheel-to-wheel multiplayer.

Execution · Pure Mechanics
Arcade racing lines and traffic-dodging reflexes on memorisable tracks.
Grid 2 leans Execution (Practiced Mastery); Test Drive 6 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.
Grid 2
Car control through corners is the entire moment-to-moment test
Test Drive 6
Cornering at speed through traffic is the entire challenge
Grid 2 leans a little harder on reads, though the gap is small.
Grid 2
Online wheel-to-wheel adds light positional prediction
Test Drive 6
AI racers follow rubber-band scripts, not minds to model
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
Test Drive 6
Traffic adds mild unpredictability to known tracks
Grid 2 leans a little harder on optimization, though the gap is small.
Grid 2
Racing lines, braking points and car selection are studyable
Test Drive 6
Learning racing lines and car choice is solvable
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.