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

Execution · Light / Casual
A meditative underwater journey about swimming through gorgeous ocean vistas.

Optimization · Light / Casual
A first-person mystery about a fire lookout in a Wyoming forest.
ABZÛ leans Execution (Light / Casual); Firewatch leans Optimization (Light / Casual).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
ABZÛ
Gentle swimming controls, no fail states
Firewatch
Walking and radio dialogue only.
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
ABZÛ
No opponents at all
Firewatch
No adaptive opposition.
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
ABZÛ
Scripted reveals, minimal surprise
Firewatch
Minor environmental discoveries.
Firewatch leans a little harder on optimization, though the gap is small.
ABZÛ
Linear path, nothing to solve
Firewatch
Few systems to optimize.
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.