head to head
Neither is simply harder - they are hard in different places. Portal demands more execution; Shin Megami Tensei V demands more variance.

Optimization · Pure Optimization
The portal-gun puzzle game whose solutions are everything.

Optimization · Pure Optimization
Open-area SMT where essence customization and Press Turn mastery turn demons into solvable math.
Portal leans Optimization (Pure Optimization); Shin Megami Tensei V leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Portal
Portal placement timing modest.
Shin Megami Tensei V
Open-field exploration, but combat is pure menus.
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Portal
No adversary modeling player.
Shin Megami Tensei V
Magatsuhi and bosses are scripted patterns.
Shin Megami Tensei V leans clearly harder on variance.
Portal
Deterministic chambers; no randomness.
Shin Megami Tensei V
Crit and negotiation RNG add manageable noise.
Shin Megami Tensei V leans a little harder on optimization, though the gap is small.
Portal
Pure puzzle-solving optimization.
Shin Megami Tensei V
Essence fusion and Press Turn builds reward perfect planning.
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.