head to head
Neither is simply harder - they are hard in different places. Portal demands more execution; SimCity 3000 Unlimited demands more variance.

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

Optimization · Pure Optimization
A city builder testing long-horizon zoning, budget and infrastructure optimization.
Portal leans Optimization (Pure Optimization); SimCity 3000 Unlimited leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Portal
Portal placement timing modest.
SimCity 3000 Unlimited
Menu clicks at a leisurely pace
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 3000 Unlimited
No opponent, only simulated citizens
SimCity 3000 Unlimited leans clearly harder on variance.
Portal
Deterministic chambers; no randomness.
SimCity 3000 Unlimited
Disasters add occasional scripted randomness
SimCity 3000 Unlimited leans a little harder on optimization, though the gap is small.
Portal
Pure puzzle-solving optimization.
SimCity 3000 Unlimited
Zoning, budgets and infrastructure balance are entirely 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.