head to head
Neither is simply harder - they are hard in different places. Ghost of a Tale demands more variance; Portal demands more optimization.

Optimization · Pure Optimization
Stealth adventure about a minstrel mouse sneaking through a rat-run fortress.

Optimization · Pure Optimization
The portal-gun puzzle game whose solutions are everything.
Ghost of a Tale leans Optimization (Pure Optimization); Portal leans Optimization (Pure Optimization).
Portal leans a little harder on execution, though the gap is small.
Ghost of a Tale
Light sneak-and-dash timing
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.
Ghost of a Tale
Guard rats patrol fixed mappable routes
Portal
No adversary modeling player.
Ghost of a Tale leans clearly harder on variance.
Ghost of a Tale
Quest items and routes hidden first time
Portal
Deterministic chambers; no randomness.
Portal leans clearly harder on optimization.
Ghost of a Tale
Costumes, routes, and quest-chain solutions
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.