head to head
Neither is simply harder - they are hard in different places. Fallout 2 demands more variance; Fallout: New Vegas demands more execution.

Optimization · Pure Optimization
A post-nuclear RPG expanded in scope where tagged builds and Gifted-trait optimization dominate.

Optimization · Pure Optimization
A faction-driven RPG-shooter where build and route planning shape every ending.
Fallout 2 leans Optimization (Pure Optimization); Fallout: New Vegas leans Optimization (Pure Optimization).
Fallout: New Vegas leans clearly harder on execution.
Fallout 2
Turn-based AP system makes twitch entirely irrelevant
Fallout: New Vegas
Shooter inputs with VATS assist
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Fallout 2
All tribal and Enclave enemies run fixed scripted combat routines
Fallout: New Vegas
Scripted factions and enemies
Fallout 2 leans clearly harder on variance.
Fallout 2
Hidden special encounters and timed events create significant unknown-information space
Fallout: New Vegas
Some loot and crit RNG
Fallout 2 leans a little harder on optimization, though the gap is small.
Fallout 2
SPECIAL and skill tagging with Gifted trait produce well-known dominant build strategies
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.