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

Optimization · Pure Optimization
The original post-apocalyptic CRPG where character builds solve the wasteland.

Optimization · Pure Optimization
A faction-driven RPG-shooter where build and route planning shape every ending.
Fallout leans Optimization (Pure Optimization); Fallout: New Vegas leans Optimization (Pure Optimization).
Fallout: New Vegas leans clearly harder on execution.
Fallout
Turn-based, no dexterity
Fallout: New Vegas
Shooter inputs with VATS assist
Fallout: New Vegas leans a little harder on reads, though the gap is small.
Fallout
No adaptive opposition
Fallout: New Vegas
Scripted factions and enemies
Fallout leans clearly harder on variance.
Fallout
To-hit rolls and random encounters
Fallout: New Vegas
Some loot and crit RNG
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Fallout
SPECIAL builds and quest routing are deeply solvable
Fallout: New Vegas
Faction routing, perks, builds heavily rewarded
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.