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.

Variance · Light / Casual
A hidden-object game about finding cats in hand-drawn cityscapes.

Optimization · Light / Casual
A first-person mystery about a fire lookout in a Wyoming forest.
100 Hidden Cats leans Variance (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.
100 Hidden Cats
Clicking cats has no timing demand
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.
100 Hidden Cats
Solo - nothing to predict
Firewatch
No adaptive opposition.
100 Hidden Cats leans a little harder on variance, though the gap is small.
100 Hidden Cats
Cat positions unknown until scanned
Firewatch
Minor environmental discoveries.
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
100 Hidden Cats
Systematic searching trivially completes it
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.