head to head
Neither is simply harder - they are hard in different places. Fallout: New Vegas demands more execution; Might & Magic: Heroes VI demands more reads.

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

Optimization · Pure Optimization
A turn-based strategy game where building the optimal army and economy decides victory.
Fallout: New Vegas leans Optimization (Pure Optimization); Might & Magic: Heroes VI leans Optimization (Pure Optimization).
Fallout: New Vegas leans clearly harder on execution.
Fallout: New Vegas
Shooter inputs with VATS assist
Might & Magic: Heroes VI
no real-time physical skill needed
Might & Magic: Heroes VI leans clearly harder on reads.
Fallout: New Vegas
Scripted factions and enemies
Might & Magic: Heroes VI
predicting human opponent strategy in PvP
Might & Magic: Heroes VI leans clearly harder on variance.
Fallout: New Vegas
Some loot and crit RNG
Might & Magic: Heroes VI
combat rolls and map resources random
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Fallout: New Vegas
Faction routing, perks, builds heavily rewarded
Might & Magic: Heroes VI
army building and economy near-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.