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

Optimization · Pure Optimization
Optimizing pair-up chains and offspring builds to steamroll deterministically scripted maps.

Optimization · Pure Optimization
The portal-gun puzzle game whose solutions are everything.
Fire Emblem Fates leans Optimization (Pure Optimization); Portal leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Fire Emblem Fates
Turn-based menu play; no hand-speed demands
Portal
Portal placement timing modest.
Fire Emblem Fates leans a little harder on reads, though the gap is small.
Fire Emblem Fates
AI follows scripted attack priorities, not player modeling
Portal
No adversary modeling player.
Fire Emblem Fates leans clearly harder on variance.
Fire Emblem Fates
Dual-strike/guard procs and hit rates inject meaningful RNG
Portal
Deterministic chambers; no randomness.
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Fire Emblem Fates
Offspring seals, asset/flaw selection, and supports are deeply planned
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.