head to head
Neither is simply harder - they are hard in different places. Empire: Total War demands more reads; Portal demands more execution.

Optimization · Pure Optimization
Gunpowder-era Total War blending empire management with line battles.

Optimization · Pure Optimization
The portal-gun puzzle game whose solutions are everything.
Empire: Total War leans Optimization (Pure Optimization); Portal leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Empire: Total War
Real-time battle micro is modest
Portal
Portal placement timing modest.
Empire: Total War leans clearly harder on reads.
Empire: Total War
Multiplayer line-battle feints exist
Portal
No adversary modeling player.
Empire: Total War leans clearly harder on variance.
Empire: Total War
Campaign AI and battle dice noise
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.
Empire: Total War
Campaign economy and line-infantry tactics are mappable
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.