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

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

Optimization · Pure Optimization
A first-person puzzle game where seeing the solution is the whole skill.
Might & Magic: Heroes VI leans Optimization (Pure Optimization); Portal 2 leans Optimization (Pure Optimization).
Portal 2 leans clearly harder on execution.
Might & Magic: Heroes VI
no real-time physical skill needed
Portal 2
Portal placement requires some precision but timing is lenient.
Might & Magic: Heroes VI leans clearly harder on reads.
Might & Magic: Heroes VI
predicting human opponent strategy in PvP
Portal 2
No opponent; puzzles are static.
Might & Magic: Heroes VI leans clearly harder on variance.
Might & Magic: Heroes VI
combat rolls and map resources random
Portal 2
Puzzles are deterministic with no randomness.
Portal 2 leans a little harder on optimization, though the gap is small.
Might & Magic: Heroes VI
army building and economy near-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.