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

Optimization · Pure Optimization
A party-based loot ARPG marching a customizable squad across Aranna against Valdis.

Optimization · Pure Optimization
A faction-driven RPG-shooter where build and route planning shape every ending.
Dungeon Siege II leans Optimization (Pure Optimization); Fallout: New Vegas leans Optimization (Pure Optimization).
Fallout: New Vegas leans clearly harder on execution.
Dungeon Siege II
Party combat mostly automates itself
Fallout: New Vegas
Shooter inputs with VATS assist
Fallout: New Vegas leans a little harder on reads, though the gap is small.
Dungeon Siege II
Enemies follow simple scripts
Fallout: New Vegas
Scripted factions and enemies
Dungeon Siege II leans clearly harder on variance.
Dungeon Siege II
Random loot and chant rewards
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.
Dungeon Siege II
Party composition, pet raising, skill tree planning
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.