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

Optimization · Pure Optimization
The portal-gun puzzle game whose solutions are everything.

Optimization · Pure Optimization
A farm-life sim solved by season routing and profit optimization.
Portal leans Optimization (Pure Optimization); Stardew Valley leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Portal
Portal placement timing modest.
Stardew Valley
Trivial timing on fishing and combat
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Portal
No adversary modeling player.
Stardew Valley
No adaptive opponent exists
Stardew Valley leans clearly harder on variance.
Portal
Deterministic chambers; no randomness.
Stardew Valley
Crop luck and geode RNG matter modestly
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Portal
Pure puzzle-solving optimization.
Stardew Valley
Routing seasons, profit loops, and bundles dominates
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.