head to head
Neither is simply harder - they are hard in different places. Lost Lands: Dark Overlord demands more variance; Portal demands more execution.

Optimization · Pure Optimization
A hidden-object adventure about solving fixed environmental and inventory puzzles.

Optimization · Pure Optimization
The portal-gun puzzle game whose solutions are everything.
Lost Lands: Dark Overlord leans Optimization (Pure Optimization); Portal leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Lost Lands: Dark Overlord
Point-and-click; no timing or aim 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.
Lost Lands: Dark Overlord
Solo with no adaptive adversary
Portal
No adversary modeling player.
Lost Lands: Dark Overlord leans clearly harder on variance.
Lost Lands: Dark Overlord
Hidden objects and clues unknown until found
Portal
Deterministic chambers; no randomness.
Portal leans clearly harder on optimization.
Lost Lands: Dark Overlord
Solving fixed inventory puzzles is the entire game
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.