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

Optimization · Pure Optimization
Expanding a self-sufficient alien colony by solving atmosphere, power, and resource chains under scripted threats.

Optimization · Pure Optimization
A faction-driven RPG-shooter where build and route planning shape every ending.
Aven Colony leans Optimization (Pure Optimization); Fallout: New Vegas leans Optimization (Pure Optimization).
Fallout: New Vegas leans clearly harder on execution.
Aven Colony
City management UI has no twitch dimension
Fallout: New Vegas
Shooter inputs with VATS assist
Fallout: New Vegas leans a little harder on reads, though the gap is small.
Aven Colony
Alien threats are scripted with no player-modelling behaviour
Fallout: New Vegas
Scripted factions and enemies
Aven Colony leans clearly harder on variance.
Aven Colony
Landing site resources and alien attack timing carry moderate RNG
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.
Aven Colony
Colony expansion, air quality chains, and tech routing are the solvable core
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.