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

Optimization · Pure Optimization
A dark-fantasy strategy RPG about building armies and winning turn-based battles.

Optimization · Pure Optimization
The portal-gun puzzle game whose solutions are everything.
Disciples: Liberation leans Optimization (Pure Optimization); Portal leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Disciples: Liberation
Turn-based combat needs no input speed
Portal
Portal placement timing modest.
Disciples: Liberation leans clearly harder on reads.
Disciples: Liberation
AI uses fixed tactics; multiplayer adds light prediction
Portal
No adversary modeling player.
Disciples: Liberation leans clearly harder on variance.
Disciples: Liberation
Damage rolls and map encounters carry randomness
Portal
Deterministic chambers; no randomness.
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Disciples: Liberation
Army composition, upgrades and tactical positioning decide battles
Portal
Pure puzzle-solving optimization.
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.