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

Optimization · Pure Optimization
The Kanto classic reworked around your starter Pikachu following you through the original journey.

Optimization · Pure Optimization
The portal-gun puzzle game whose solutions are everything.
Pokemon Yellow Version leans Optimization (Pure Optimization); Portal leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Pokemon Yellow Version
Turn-based menus; no dexterity required
Portal
Portal placement timing modest.
Pokemon Yellow Version leans clearly harder on reads.
Pokemon Yellow Version
Trainers are scripted; link battles vs friends optional
Portal
No adversary modeling player.
Pokemon Yellow Version leans clearly harder on variance.
Pokemon Yellow Version
Catch rates, encounters, and crits add luck
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.
Pokemon Yellow Version
Team building, type matchups, and routing are the game
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.