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

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

Optimization · Pure Optimization
Exploiting the press-turn system through demon fusion and weakness math.
Portal leans Optimization (Pure Optimization); Shin Megami Tensei: Nocturne leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Portal
Portal placement timing modest.
Shin Megami Tensei: Nocturne
Turn-based menus carry zero dexterity demands
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: Nocturne
Brutal bosses still run fixed scripts
Shin Megami Tensei: Nocturne leans clearly harder on variance.
Portal
Deterministic chambers; no randomness.
Shin Megami Tensei: Nocturne
Unknown boss gimmicks and fusion luck sting first attempts
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Portal
Pure puzzle-solving optimization.
Shin Megami Tensei: Nocturne
Press-turn exploitation and fusion planning decide everything
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.