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 · Pure Optimization
A sandbox city builder that is pure traffic and zoning optimization.

Optimization · Pure Optimization
Sandbox city-builder where everything bends to optimization.
Cities: Skylines leans Optimization (Pure Optimization); Cities: Skylines II leans Optimization (Pure Optimization).
Cities: Skylines II leans a little harder on execution, though the gap is small.
Cities: Skylines
No reflex demands at all.
Cities: Skylines II
no mechanical demand
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Cities: Skylines
No adaptive opponent exists in the sandbox.
Cities: Skylines II
no adversary
Cities: Skylines II leans a little harder on variance, though the gap is small.
Cities: Skylines
Minor disaster events if enabled.
Cities: Skylines II
minor sim noise
Cities: Skylines leans a little harder on optimization, though the gap is small.
Cities: Skylines
Traffic flow, zoning, and budgets are fully solvable.
Cities: Skylines II
traffic, zoning, budgets solvable
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.