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

Optimization · Pure Optimization
Cyberpunk point-and-click adventure through a dystopian Australian mega-city.

Optimization · Pure Optimization
The portal-gun puzzle game whose solutions are everything.
Beneath a Steel Sky leans Optimization (Pure Optimization); Portal leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Beneath a Steel Sky
Point-and-click; no dexterity required
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.
Beneath a Steel Sky
No opponent; scripted characters
Portal
No adversary modeling player.
Beneath a Steel Sky leans clearly harder on variance.
Beneath a Steel Sky
First-run puzzle surprises only
Portal
Deterministic chambers; no randomness.
Portal leans clearly harder on optimization.
Beneath a Steel Sky
Inventory puzzle chains are 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.