head to head
Neither is simply harder - they are hard in different places. Heroes of Might and Magic IV demands more variance; Portal demands more execution.

Optimization · Pure Optimization
A turn-based strategy game where building the optimal army and economy decides victory.

Optimization · Pure Optimization
The portal-gun puzzle game whose solutions are everything.
Heroes of Might and Magic IV leans Optimization (Pure Optimization); Portal leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Heroes of Might and Magic IV
no real-time physical skill needed
Portal
Portal placement timing modest.
Heroes of Might and Magic IV leans clearly harder on reads.
Heroes of Might and Magic IV
predicting human opponent strategy in PvP
Portal
No adversary modeling player.
Heroes of Might and Magic IV leans clearly harder on variance.
Heroes of Might and Magic IV
combat rolls and map resources random
Portal
Deterministic chambers; no randomness.
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Heroes of Might and Magic IV
army building and economy near-solvable
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.