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

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

Optimization · Pure Optimization
Balancing town supply chains against mine logistics in a solvable economy.
Portal leans Optimization (Pure Optimization); SteamWorld Build leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Portal
Portal placement timing modest.
SteamWorld Build
Placing buildings has no dexterity component
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.
SteamWorld Build
Creature attacks are scripted waves, not an adapting mind
SteamWorld Build leans clearly harder on variance.
Portal
Deterministic chambers; no randomness.
SteamWorld Build
Resource layout below ground is unknown until dug
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.
SteamWorld Build
Supply chains, citizen needs and mine logistics are the entire test
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.