head to head
Neither is simply harder - they are hard in different places. Portal 2 demands more execution; SteamWorld Build demands more variance.

Optimization · Pure Optimization
A first-person puzzle game where seeing the solution is the whole skill.

Optimization · Pure Optimization
Balancing town supply chains against mine logistics in a solvable economy.
Portal 2 leans Optimization (Pure Optimization); SteamWorld Build leans Optimization (Pure Optimization).
Portal 2 leans clearly harder on execution.
Portal 2
Portal placement requires some precision but timing is lenient.
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 2
No opponent; puzzles are static.
SteamWorld Build
Creature attacks are scripted waves, not an adapting mind
SteamWorld Build leans clearly harder on variance.
Portal 2
Puzzles are deterministic with no randomness.
SteamWorld Build
Resource layout below ground is unknown until dug
Portal 2 leans a little harder on optimization, though the gap is small.
Portal 2
Solving the puzzle's logic IS the entire game.
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.