head to head
Neither is simply harder - they are hard in different places. Fallout: New Vegas demands more execution; Old World demands more reads.

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

Optimization · Pure Optimization
Classical 4X with dynastic events and order economy.
Fallout: New Vegas leans Optimization (Pure Optimization); Old World leans Optimization (Pure Optimization).
Fallout: New Vegas leans clearly harder on execution.
Fallout: New Vegas
Shooter inputs with VATS assist
Old World
turn-based, no twitch
Old World leans clearly harder on reads.
Fallout: New Vegas
Scripted factions and enemies
Old World
AI scripted, some MP
Old World leans clearly harder on variance.
Fallout: New Vegas
Some loot and crit RNG
Old World
event cards, character traits
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
Old World
orders, families, tech routing
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.