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 · Light / Casual
An interactive wordless short story with essentially no skill test.

Optimization · Light / Casual
A first-person mystery about a fire lookout in a Wyoming forest.
A Bird Story 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.
A Bird Story
Walking through scenes is the only input
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.
A Bird Story
No opponent of any kind
Firewatch
No adaptive opposition.
Firewatch leans a little harder on variance, though the gap is small.
A Bird Story
Wholly linear with nothing hidden mechanically
Firewatch
Minor environmental discoveries.
Firewatch leans a little harder on optimization, though the gap is small.
A Bird Story
Nothing to solve; it is a told story
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.