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

Optimization · Pure Optimization
Building a team and exploiting type-matchup math against scripted trainers.

Optimization · Pure Optimization
The portal-gun puzzle game whose solutions are everything.
Pokémon LeafGreen Version leans Optimization (Pure Optimization); Portal leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Pokémon LeafGreen Version
Menu-driven turn battles demand no physical skill
Portal
Portal placement timing modest.
Pokémon LeafGreen Version leans a little harder on reads, though the gap is small.
Pokémon LeafGreen Version
Trainers are scripted; link battles are a side mode
Portal
No adversary modeling player.
Pokémon LeafGreen Version leans clearly harder on variance.
Pokémon LeafGreen Version
Wild encounters, catch rolls and crits add mappable RNG
Portal
Deterministic chambers; no randomness.
Portal leans a little harder on optimization, though the gap is small.
Pokémon LeafGreen Version
Type matchups and team building are the solvable core
Portal
Pure puzzle-solving optimization.
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.