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

Execution · Practiced Mastery
Precise shot aiming plus planning the run-out several balls ahead.

Execution · Practiced Mastery
Cornering execution over memorisable circuits, with light prediction in wheel-to-wheel multiplayer.
8 Ball Pool leans Execution (Practiced Mastery); Grid 2 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.
8 Ball Pool
A perfect aim-line cheat wins nearly every rack outright
Grid 2
Car control through corners is the entire moment-to-moment test
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
8 Ball Pool
Opponents alternate turns; little adaptive interaction to predict
Grid 2
Online wheel-to-wheel adds light positional prediction
8 Ball Pool leans a little harder on variance, though the gap is small.
8 Ball Pool
Break spreads add some randomness to each rack
Grid 2
Tracks are fixed and fully visible
8 Ball Pool leans a little harder on optimization, though the gap is small.
8 Ball Pool
Position play and run-out planning separate good players
Grid 2
Racing lines, braking points and car selection are studyable
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.