head to head
Neither is simply harder - they are hard in different places. Portal demands more execution; Sid Meier's Civilization II demands more variance.

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

Optimization · Pure Optimization
The 1996 landmark 4X where you guide a civilization from antiquity to the space race.
Portal leans Optimization (Pure Optimization); Sid Meier's Civilization II leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Portal
Portal placement timing modest.
Sid Meier's Civilization II
Turn-based clicking only
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.
Sid Meier's Civilization II
AI civs are exploitable scripts, not adaptive rivals
Sid Meier's Civilization II leans clearly harder on variance.
Portal
Deterministic chambers; no randomness.
Sid Meier's Civilization II
Random maps and hut rolls inject early uncertainty
Sid Meier's Civilization II leans a little harder on optimization, though the gap is small.
Portal
Pure puzzle-solving optimization.
Sid Meier's Civilization II
City spacing, trade routes and tech paths are deeply 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.