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

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

Optimization · Pure Optimization
Building a team and exploiting type-matchup math against scripted trainers.
Fallout: New Vegas leans Optimization (Pure Optimization); Pokémon LeafGreen Version leans Optimization (Pure Optimization).
Fallout: New Vegas leans clearly harder on execution.
Fallout: New Vegas
Shooter inputs with VATS assist
Pokémon LeafGreen Version
Menu-driven turn battles demand no physical skill
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
Pokémon LeafGreen Version
Trainers are scripted; link battles are a side mode
Pokémon LeafGreen Version leans clearly harder on variance.
Fallout: New Vegas
Some loot and crit RNG
Pokémon LeafGreen Version
Wild encounters, catch rolls and crits add mappable RNG
Fallout: New Vegas leans a little harder on optimization, though the gap is small.
Fallout: New Vegas
Faction routing, perks, builds heavily rewarded
Pokémon LeafGreen Version
Type matchups and team building are the solvable core
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.