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

Optimization · Pure Optimization
A turn-based RPG where system mastery and build optimization decide every fight.

Optimization · Pure Optimization
A faction-driven RPG-shooter where build and route planning shape every ending.
Divinity: Original Sin - Enhanced Edition leans Optimization (Pure Optimization); Fallout: New Vegas leans Optimization (Pure Optimization).
Fallout: New Vegas leans clearly harder on execution.
Divinity: Original Sin - Enhanced Edition
no real-time physical skill, turn-based
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.
Divinity: Original Sin - Enhanced Edition
co-op has light unpredictability, mostly solo
Fallout: New Vegas
Scripted factions and enemies
Divinity: Original Sin - Enhanced Edition leans clearly harder on variance.
Divinity: Original Sin - Enhanced Edition
dice rolls and loot are randomized
Fallout: New Vegas
Some loot and crit RNG
Divinity: Original Sin - Enhanced Edition leans a little harder on optimization, though the gap is small.
Divinity: Original Sin - Enhanced Edition
builds, combos, and tactics near-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.