head to head
Neither is simply harder - they are hard in different places. Pokémon SoulSilver Version demands more variance; Portal 2 demands more execution.

Optimization · Pure Optimization
Plannable type-matchup progression through two regions of fixed trainers.

Optimization · Pure Optimization
A first-person puzzle game where seeing the solution is the whole skill.
Pokémon SoulSilver Version leans Optimization (Pure Optimization); Portal 2 leans Optimization (Pure Optimization).
Portal 2 leans clearly harder on execution.
Pokémon SoulSilver Version
Menu-driven battles have no physical component
Portal 2
Portal placement requires some precision but timing is lenient.
Pokémon SoulSilver Version leans a little harder on reads, though the gap is small.
Pokémon SoulSilver Version
Trainer AI is scripted; human battles are an optional sideline
Portal 2
No opponent; puzzles are static.
Pokémon SoulSilver Version leans clearly harder on variance.
Pokémon SoulSilver Version
Catch rates, crits and encounters are bounded randomness
Portal 2
Puzzles are deterministic with no randomness.
Portal 2 leans a little harder on optimization, though the gap is small.
Pokémon SoulSilver Version
Team building against known gyms is fully plannable
Portal 2
Solving the puzzle's logic IS the entire game.
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.