head to head
Neither is simply harder - they are hard in different places. Portal demands more execution; Space Rangers 2: Dominators demands more variance.

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

Optimization · Pure Optimization
A sprawling space RPG-strategy hybrid where mastering trade, upgrades, and fleet building trumps everything else.
Portal leans Optimization (Pure Optimization); Space Rangers 2: Dominators leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Portal
Portal placement timing modest.
Space Rangers 2: Dominators
real-time combat pauses, minimal twitch skill needed
Space Rangers 2: Dominators leans a little harder on reads, though the gap is small.
Portal
No adversary modeling player.
Space Rangers 2: Dominators
no adaptive human opponent in single-player sandbox
Space Rangers 2: Dominators leans clearly harder on variance.
Portal
Deterministic chambers; no randomness.
Space Rangers 2: Dominators
random encounters, text quests, galaxy generation
Space Rangers 2: Dominators leans a little harder on optimization, though the gap is small.
Portal
Pure puzzle-solving optimization.
Space Rangers 2: Dominators
deep economy, tech tree, fleet build all solvable
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.