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

Optimization · Pure Optimization
Low-pressure production-chain and layout optimization in a fairytale wrapper.

Optimization · Pure Optimization
The portal-gun puzzle game whose solutions are everything.
Fabledom leans Optimization (Pure Optimization); Portal leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Fabledom
Placing buildings at a cozy pace, no 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.
Fabledom
Neighboring rulers follow scripted diplomacy states
Portal
No adversary modeling player.
Fabledom leans clearly harder on variance.
Fabledom
Events and neighbor moods add gentle randomness
Portal
Deterministic chambers; no randomness.
Portal leans a little harder on optimization, though the gap is small.
Fabledom
Production chains and settlement layout are the real 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.