head to head
Neither is simply harder - they are hard in different places. Hearts of Iron IV demands more reads; Portal demands more execution.

Optimization · Pure Optimization
WWII grand strategy: templates, production, and focus paths.

Optimization · Pure Optimization
The portal-gun puzzle game whose solutions are everything.
Hearts of Iron IV leans Optimization (Pure Optimization); Portal leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Hearts of Iron IV
pausable, micromanagement-heavy
Portal
Portal placement timing modest.
Hearts of Iron IV leans clearly harder on reads.
Hearts of Iron IV
PvP partly, mostly scripted AI
Portal
No adversary modeling player.
Hearts of Iron IV leans clearly harder on variance.
Hearts of Iron IV
focus tree branches, events
Portal
Deterministic chambers; no randomness.
Hearts of Iron IV leans a little harder on optimization, though the gap is small.
Hearts of Iron IV
division templates and production solved
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.