head to head
Neither is simply harder - they are hard in different places. Persona 3 Portable demands more variance; Portal 2 demands more execution.

Optimization · Pure Optimization
Optimising a strict social calendar and fusion economy against scripted dungeon combat.

Optimization · Pure Optimization
A first-person puzzle game where seeing the solution is the whole skill.
Persona 3 Portable leans Optimization (Pure Optimization); Portal 2 leans Optimization (Pure Optimization).
Portal 2 leans clearly harder on execution.
Persona 3 Portable
Menu-driven combat needs no physical skill
Portal 2
Portal placement requires some precision but timing is lenient.
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Persona 3 Portable
Shadows follow scripted weakness patterns, never adapt
Portal 2
No opponent; puzzles are static.
Persona 3 Portable leans clearly harder on variance.
Persona 3 Portable
Tartarus layouts and fusion results carry randomness
Portal 2
Puzzles are deterministic with no randomness.
Portal 2 leans a little harder on optimization, though the gap is small.
Persona 3 Portable
Calendar scheduling and persona fusion form a solvable system
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.