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

Optimization · Pure Optimization
A stealth puzzle game where meticulously planning the perfect assassination route is the whole point.

Optimization · Pure Optimization
The portal-gun puzzle game whose solutions are everything.
Hitman leans Optimization (Pure Optimization); Portal leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Hitman
minimal twitch skill, stealth-focused
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.
Hitman
guard patrols scripted, no adaptation
Portal
No adversary modeling player.
Hitman leans clearly harder on variance.
Hitman
some random guard behavior variance
Portal
Deterministic chambers; no randomness.
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Hitman
planning routes and disguises is fully solvable
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.