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

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

Optimization · Pure Optimization
Colonize Mars by stacking dome layouts and supply chains.
Portal leans Optimization (Pure Optimization); Surviving Mars leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Portal
Portal placement timing modest.
Surviving Mars
pausable, no reflex
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.
Surviving Mars
no adversary
Surviving Mars leans clearly harder on variance.
Portal
Deterministic chambers; no randomness.
Surviving Mars
disasters, sponsor traits
Surviving Mars leans a little harder on optimization, though the gap is small.
Portal
Pure puzzle-solving optimization.
Surviving Mars
dome layouts and supply chains
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.