head to head
Neither is simply harder - they are hard in different places. Cities: Skylines demands more optimization; Tropico 6 demands more variance.

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

Optimization · Pure Optimization
Banana-republic management; juggle factions and the treasury.
Cities: Skylines leans Optimization (Pure Optimization); Tropico 6 leans Optimization (Pure Optimization).
Tropico 6 leans a little harder on execution, though the gap is small.
Cities: Skylines
No reflex demands at all.
Tropico 6
pausable management
Tropico 6 leans a little harder on reads, though the gap is small.
Cities: Skylines
No adaptive opponent exists in the sandbox.
Tropico 6
factions scripted not adaptive
Tropico 6 leans clearly harder on variance.
Cities: Skylines
Minor disaster events if enabled.
Tropico 6
election events, disasters
Cities: Skylines leans clearly harder on optimization.
Cities: Skylines
Traffic flow, zoning, and budgets are fully solvable.
Tropico 6
economy and faction balance 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.