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

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

Optimization · Pure Optimization
The original post-apocalyptic demon-fusing dungeon crawler where preparation and fusion knowledge decide survival.
Fallout: New Vegas leans Optimization (Pure Optimization); Shin Megami Tensei leans Optimization (Pure Optimization).
Fallout: New Vegas leans clearly harder on execution.
Fallout: New Vegas
Shooter inputs with VATS assist
Shin Megami Tensei
First-person menu battles; trivial inputs.
Fallout: New Vegas leans a little harder on reads, though the gap is small.
Fallout: New Vegas
Scripted factions and enemies
Shin Megami Tensei
Demon negotiation is random, not adaptive.
Shin Megami Tensei leans clearly harder on variance.
Fallout: New Vegas
Some loot and crit RNG
Shin Megami Tensei
Negotiation rolls, ambushes, and moon-phase RNG bite hard.
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
Shin Megami Tensei
Fusion charts, alignment routes, and weakness mapping are 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.