head to head
Neither is simply harder - they are hard in different places. Portal demands more execution; SimCity (2013) demands more variance.

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

Optimization · Pure Optimization
A small-plot city builder where specialization economics and traffic engineering decide success.
Portal leans Optimization (Pure Optimization); SimCity (2013) leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Portal
Portal placement timing modest.
SimCity (2013)
Mouse planning; no dexterity demands.
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 (2013)
No opponent; multiplayer regions cooperate.
SimCity (2013) leans clearly harder on variance.
Portal
Deterministic chambers; no randomness.
SimCity (2013)
Agent simulation quirks behave unpredictably.
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Portal
Pure puzzle-solving optimization.
SimCity (2013)
Specialization chains and traffic layouts reward systematic planning.
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.