head to head
Neither is simply harder - they are hard in different places. Hexic HD demands more variance; Portal 2 demands more optimization.

Optimization · Pure Optimization
Patient combinatorial planning to engineer rare tile formations against drop randomness.

Optimization · Pure Optimization
A first-person puzzle game where seeing the solution is the whole skill.
Hexic HD leans Optimization (Pure Optimization); Portal 2 leans Optimization (Pure Optimization).
Portal 2 leans a little harder on execution, though the gap is small.
Hexic HD
Unhurried rotations - minimal dexterity needed
Portal 2
Portal placement requires some precision but timing is lenient.
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Hexic HD
Solo board with no adversary
Portal 2
No opponent; puzzles are static.
Hexic HD leans clearly harder on variance.
Hexic HD
New tile colors drop semi-randomly into plans
Portal 2
Puzzles are deterministic with no randomness.
Portal 2 leans clearly harder on optimization.
Hexic HD
Engineering starflowers and black pearls is deep planning
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.