head to head
Neither is simply harder - they are hard in different places. Portal demands more execution; The Battle for Wesnoth demands more variance.

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

Optimization · Pure Optimization
An open-source hex strategy game famous for its luck-laced combat rolls.
Portal leans Optimization (Pure Optimization); The Battle for Wesnoth leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Portal
Portal placement timing modest.
The Battle for Wesnoth
Turn-based hex moves, no dexterity.
The Battle for Wesnoth leans clearly harder on reads.
Portal
No adversary modeling player.
The Battle for Wesnoth
Multiplayer opponents adapt; campaigns scripted.
The Battle for Wesnoth leans clearly harder on variance.
Portal
Deterministic chambers; no randomness.
The Battle for Wesnoth
To-hit rolls swing battles constantly.
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.
The Battle for Wesnoth
Unit matchups, terrain, and economy solvable.
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.