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

Optimization · Pure Optimization
Solving fixed gravity-bending puzzles inside a deterministic nightmare.

Optimization · Pure Optimization
The portal-gun puzzle game whose solutions are everything.
Darq leans Optimization (Pure Optimization); Portal leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Darq
Slow stealth walking; minimal timing demands
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.
Darq
Nightmare enemies patrol fixed scripted routes
Portal
No adversary modeling player.
Darq leans clearly harder on variance.
Darq
Puzzle solutions hidden only until first discovery
Portal
Deterministic chambers; no randomness.
Portal leans a little harder on optimization, though the gap is small.
Darq
Gravity-bending puzzle logic is the entire test
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.