head to head
Neither is simply harder - they are hard in different places. Portal demands more execution; Zork III: The Dungeon Master demands more variance.

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

Optimization · Pure Optimization
A text adventure testing puzzle deduction and mapping hidden dungeon logic.
Portal leans Optimization (Pure Optimization); Zork III: The Dungeon Master leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Portal
Portal placement timing modest.
Zork III: The Dungeon Master
Typing commands has no physical ceiling
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Portal
No adversary modeling player.
Zork III: The Dungeon Master
No adaptive opponent anywhere in the dungeon
Zork III: The Dungeon Master leans clearly harder on variance.
Portal
Deterministic chambers; no randomness.
Zork III: The Dungeon Master
Hidden rooms and timed events punish unmapped exploration
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.
Zork III: The Dungeon Master
Puzzle solutions and routing are fully solvable with knowledge
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.