head to head
Neither is simply harder - they are hard in different places. Fallout: New Vegas demands more execution; Sid Meier's Civilization IV: Colonization demands more variance.

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

Optimization · Pure Optimization
Optimising colonial economy and revolution timing on an unknown map.
Fallout: New Vegas leans Optimization (Pure Optimization); Sid Meier's Civilization IV: Colonization leans Optimization (Pure Optimization).
Fallout: New Vegas leans clearly harder on execution.
Fallout: New Vegas
Shooter inputs with VATS assist
Sid Meier's Civilization IV: Colonization
Turn-based clicks - no physical layer
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Fallout: New Vegas
Scripted factions and enemies
Sid Meier's Civilization IV: Colonization
AI rivals follow mappable tendencies
Sid Meier's Civilization IV: Colonization leans clearly harder on variance.
Fallout: New Vegas
Some loot and crit RNG
Sid Meier's Civilization IV: Colonization
Map generation and exploration hide early information
Sid Meier's Civilization IV: Colonization leans a little harder on optimization, though the gap is small.
Fallout: New Vegas
Faction routing, perks, builds heavily rewarded
Sid Meier's Civilization IV: Colonization
Colony economy, specialists and independence timing are deeply solvable
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.