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

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

Optimization · Pure Optimization
Handheld SMT where smirk RNG stings but fusion planning and weakness knowledge carry you.
Portal leans Optimization (Pure Optimization); Shin Megami Tensei IV leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Portal
Portal placement timing modest.
Shin Megami Tensei IV
Menu combat with no execution ceiling.
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 IV
Demon negotiation is random personality scripts.
Shin Megami Tensei IV leans clearly harder on variance.
Portal
Deterministic chambers; no randomness.
Shin Megami Tensei IV
Smirk procs and negotiation RNG swing fights hard.
Shin Megami Tensei IV leans a little harder on optimization, though the gap is small.
Portal
Pure puzzle-solving optimization.
Shin Megami Tensei IV
Fusion apps, weakness charts, and builds solve the 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.