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

Optimization · Light / Casual
A first-person mystery about a fire lookout in a Wyoming forest.

Optimization · Light / Casual
A meta-narrative walking sim about defying or obeying a scripted narrator.
Firewatch leans Optimization (Light / Casual); The Stanley Parable: Ultra Deluxe leans Optimization (Light / Casual).
Firewatch leans a little harder on execution, though the gap is small.
Firewatch
Walking and radio dialogue only.
The Stanley Parable: Ultra Deluxe
Walking and clicking, no mechanical demand.
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Firewatch
No adaptive opposition.
The Stanley Parable: Ultra Deluxe
Narrator is scripted, not adaptive.
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Firewatch
Minor environmental discoveries.
The Stanley Parable: Ultra Deluxe
Hidden endings to discover.
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Firewatch
Few systems to optimize.
The Stanley Parable: Ultra Deluxe
Mapping ending tree rewards completionists.
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.