head to head
Neither is simply harder - they are hard in different places. Nancy Drew: Midnight in Salem demands more variance; Portal demands more execution.

Optimization · Pure Optimization
A point-and-click mystery testing deduction and puzzle-solving to solve a case.

Optimization · Pure Optimization
The portal-gun puzzle game whose solutions are everything.
Nancy Drew: Midnight in Salem leans Optimization (Pure Optimization); Portal leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Nancy Drew: Midnight in Salem
Point-and-click interaction, no twitch demand
Portal
Portal placement timing modest.
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Nancy Drew: Midnight in Salem
Scripted suspects, no adaptive opponent
Portal
No adversary modeling player.
Nancy Drew: Midnight in Salem leans clearly harder on variance.
Nancy Drew: Midnight in Salem
Hidden clues revealed by investigation, then mapped
Portal
Deterministic chambers; no randomness.
Portal leans a little harder on optimization, though the gap is small.
Nancy Drew: Midnight in Salem
Solving puzzles and assembling clues is the core
Portal
Pure puzzle-solving optimization.
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.