head to head
Neither is simply harder - they are hard in different places. Portal 2 demands more optimization; Squad Assault: Second Wave demands more variance.

Optimization · Pure Optimization
A first-person puzzle game where seeing the solution is the whole skill.
Optimization · Pure Optimization
A tactical squad-based shooter set in modern warfare scenarios.
Portal 2 leans Optimization (Pure Optimization); Squad Assault: Second Wave leans Optimization (Pure Optimization).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Portal 2
Portal placement requires some precision but timing is lenient.
Squad Assault: Second Wave
tactical shooting inputs, moderate demand
Squad Assault: Second Wave leans a little harder on reads, though the gap is small.
Portal 2
No opponent; puzzles are static.
Squad Assault: Second Wave
scripted enemy AI squads, not adaptive
Squad Assault: Second Wave leans clearly harder on variance.
Portal 2
Puzzles are deterministic with no randomness.
Squad Assault: Second Wave
random mission events and encounters
Portal 2 leans clearly harder on optimization.
Portal 2
Solving the puzzle's logic IS the entire game.
Squad Assault: Second Wave
squad tactics and loadout 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.