head to head
Neither is simply harder - they are hard in different places. Heroes of Might and Magic III demands more variance; Portal 2 demands more execution.

Optimization · Pure Optimization
Legendary turn-based strategy of castle economies, hero armies, and tile-perfect logistics.

Optimization · Pure Optimization
A first-person puzzle game where seeing the solution is the whole skill.
Heroes of Might and Magic III leans Optimization (Pure Optimization); Portal 2 leans Optimization (Pure Optimization).
Portal 2 leans clearly harder on execution.
Heroes of Might and Magic III
Turn-based; zero dexterity
Portal 2
Portal placement requires some precision but timing is lenient.
Heroes of Might and Magic III leans clearly harder on reads.
Heroes of Might and Magic III
Human multiplayer adds bluffing; AI is mappable
Portal 2
No opponent; puzzles are static.
Heroes of Might and Magic III leans clearly harder on variance.
Heroes of Might and Magic III
Map fog, creature banks, and morale rolls
Portal 2
Puzzles are deterministic with 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 III
Hero chains, build orders, and combat math are deeply solvable
Portal 2
Solving the puzzle's logic IS the entire 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.