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

Optimization · Pure Optimization
A faction-driven RPG-shooter where build and route planning shape every ending.

Optimization · Pure Optimization
A text adventure testing puzzle deduction and mapping hidden dungeon logic.
Fallout: New Vegas leans Optimization (Pure Optimization); Zork III: The Dungeon Master leans Optimization (Pure Optimization).
Fallout: New Vegas leans clearly harder on execution.
Fallout: New Vegas
Shooter inputs with VATS assist
Zork III: The Dungeon Master
Typing commands has no physical ceiling
Fallout: New Vegas leans a little harder on reads, though the gap is small.
Fallout: New Vegas
Scripted factions and enemies
Zork III: The Dungeon Master
No adaptive opponent anywhere in the dungeon
Zork III: The Dungeon Master leans clearly harder on variance.
Fallout: New Vegas
Some loot and crit RNG
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.
Fallout: New Vegas
Faction routing, perks, builds heavily rewarded
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.