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

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

Optimization · Pure Optimization
The portal-gun puzzle game whose solutions are everything.
Hexic HD leans Optimization (Pure Optimization); Portal leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Hexic HD
Unhurried rotations - minimal dexterity needed
Portal
Portal placement timing modest.
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
No adversary modeling player.
Hexic HD leans clearly harder on variance.
Hexic HD
New tile colors drop semi-randomly into plans
Portal
Deterministic chambers; no randomness.
Portal leans a little harder on optimization, though the gap is small.
Hexic HD
Engineering starflowers and black pearls is deep planning
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.