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

Optimization · Pure Optimization
Classic Roman city builder dominated by mastering its walker-based logistics.

Optimization · Pure Optimization
The portal-gun puzzle game whose solutions are everything.
Caesar III leans Optimization (Pure Optimization); Portal leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Caesar III
Mouse-driven building, no pressure
Portal
Portal placement timing modest.
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Caesar III
Invasions are scheduled, not adaptive
Portal
No adversary modeling player.
Caesar III leans clearly harder on variance.
Caesar III
Minor walker pathing unpredictability
Portal
Deterministic chambers; no randomness.
Caesar III leans a little harder on optimization, though the gap is small.
Caesar III
Housing blocks and walker logistics are solvable systems
Portal
Pure puzzle-solving optimization.
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.