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

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

Optimization · Pure Optimization
A first-person farming game about wrangling adorable slimes for plorts.
Portal leans Optimization (Pure Optimization); Slime Rancher leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Portal
Portal placement timing modest.
Slime Rancher
Vacuum aiming is gentle.
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.
Slime Rancher
Slimes follow scripted behaviors.
Slime Rancher leans clearly harder on variance.
Portal
Deterministic chambers; no randomness.
Slime Rancher
Plort prices and slime spawns vary.
Portal leans a little harder on optimization, though the gap is small.
Portal
Pure puzzle-solving optimization.
Slime Rancher
Ranch layout and economy reward planning.
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.