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

Optimization · Pure Optimization
Turn-based team-building optimization with collection RNG and optional human battles.

Optimization · Pure Optimization
The portal-gun puzzle game whose solutions are everything.
Pokémon Ultra Moon leans Optimization (Pure Optimization); Portal leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Pokémon Ultra Moon
Turn-based menus require no dexterity
Portal
Portal placement timing modest.
Pokémon Ultra Moon leans clearly harder on reads.
Pokémon Ultra Moon
Only human link battles involve predicting switches
Portal
No adversary modeling player.
Pokémon Ultra Moon leans clearly harder on variance.
Pokémon Ultra Moon
Wild encounters, IVs and catch rolls add randomness
Portal
Deterministic chambers; no randomness.
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Pokémon Ultra Moon
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.