head to head
Neither is simply harder - they are hard in different places. Portal demands more execution; SimCity 4 demands more variance.

Optimization · Pure Optimization
The portal-gun puzzle game whose solutions are everything.

Optimization · Pure Optimization
Optimizing regional economics and city services to sustain growth without collapsing your budget.
Portal leans Optimization (Pure Optimization); SimCity 4 leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Portal
Portal placement timing modest.
SimCity 4
Point-and-click placement; zero reflex requirement
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Portal
No adversary modeling player.
SimCity 4
Solo sandbox with no adversarial pressure
SimCity 4 leans clearly harder on variance.
Portal
Deterministic chambers; no randomness.
SimCity 4
Neighbor city interactions and demand curves add unpredictability
SimCity 4 leans a little harder on optimization, though the gap is small.
Portal
Pure puzzle-solving optimization.
SimCity 4
Budget, ordinances, and transport networks are the core puzzle
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.