head to head
Neither is simply harder - they are hard in different places. Portal demands more execution; Star Trek Fleet Command demands more reads.

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

Optimization · Pure Optimization
A timer-and-spreadsheet 4X where officer math and resource pipelines decide everything.
Portal leans Optimization (Pure Optimization); Star Trek Fleet Command leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Portal
Portal placement timing modest.
Star Trek Fleet Command
Menu taps and timers - no hands required
Star Trek Fleet Command leans clearly harder on reads.
Portal
No adversary modeling player.
Star Trek Fleet Command
Rival alliances raid and retaliate, mildly worth predicting
Star Trek Fleet Command leans clearly harder on variance.
Portal
Deterministic chambers; no randomness.
Star Trek Fleet Command
Loot rolls and unscouted systems add hidden information
Portal leans a little harder on optimization, though the gap is small.
Portal
Pure puzzle-solving optimization.
Star Trek Fleet Command
Officer synergies, ship tiers and resource pipelines are the real game
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.