head to head
Neither is simply harder - they are hard in different places. Fallout: New Vegas demands more execution; Persona 3 Portable demands more variance.

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

Optimization · Pure Optimization
Optimising a strict social calendar and fusion economy against scripted dungeon combat.
Fallout: New Vegas leans Optimization (Pure Optimization); Persona 3 Portable leans Optimization (Pure Optimization).
Fallout: New Vegas leans clearly harder on execution.
Fallout: New Vegas
Shooter inputs with VATS assist
Persona 3 Portable
Menu-driven combat needs no physical skill
Fallout: New Vegas leans a little harder on reads, though the gap is small.
Fallout: New Vegas
Scripted factions and enemies
Persona 3 Portable
Shadows follow scripted weakness patterns, never adapt
Persona 3 Portable leans clearly harder on variance.
Fallout: New Vegas
Some loot and crit RNG
Persona 3 Portable
Tartarus layouts and fusion results carry randomness
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
Persona 3 Portable
Calendar scheduling and persona fusion form a solvable system
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.