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.

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

Optimization · Pure Optimization
An elegant hex-grid logic puzzler that is always solvable by deduction alone, never guessing.
Cities: Skylines leans Optimization (Pure Optimization); Hexcells leans Optimization (Pure Optimization).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Cities: Skylines
No reflex demands at all.
Hexcells
Clicking hexes calmly; no physical test.
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.
Hexcells
No opponent exists.
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Cities: Skylines
Minor disaster events if enabled.
Hexcells
Designed to need zero guessing; full information.
Cities: Skylines leans a little harder on optimization, though the gap is small.
Cities: Skylines
Traffic flow, zoning, and budgets are fully solvable.
Hexcells
Minesweeper-style pure deduction is the entire game.
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.