head to head
Neither is simply harder - they are hard in different places. Deadly Premonition demands more variance; Portal demands more execution.

Optimization · Pure Optimization
A famously janky open-world horror mystery where FBI agent Francis York Morgan investigates a small-town murder.

Optimization · Pure Optimization
The portal-gun puzzle game whose solutions are everything.
Deadly Premonition leans Optimization (Pure Optimization); Portal leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Deadly Premonition
Clunky combat is forgiving and rarely the obstacle
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.
Deadly Premonition
Shadows and the Raincoat Killer follow fixed scripts
Portal
No adversary modeling player.
Deadly Premonition leans clearly harder on variance.
Deadly Premonition
Town schedules and side content are unknown until discovered
Portal
Deterministic chambers; no randomness.
Portal leans a little harder on optimization, though the gap is small.
Deadly Premonition
Time management, schedules, and routing dominate completion
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.