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

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

Optimization · Pure Optimization
A turn-based tactics game of dice-driven squad combat in cramped corridors.
Portal leans Optimization (Pure Optimization); Space Hulk: Tactics leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Portal
Portal placement timing modest.
Space Hulk: Tactics
Turn-based, no dexterity needed
Space Hulk: Tactics leans clearly harder on reads.
Portal
No adversary modeling player.
Space Hulk: Tactics
Multiplayer involves some reading of human commanders
Space Hulk: Tactics leans clearly harder on variance.
Portal
Deterministic chambers; no randomness.
Space Hulk: Tactics
Dice rolls and card draws inject combat randomness
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.
Space Hulk: Tactics
Positioning, action economy and card play decide battles
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.