head to head
Neither is simply harder - they are hard in different places. Portal 2 demands more optimization; South Park Let's Go Tower Defense Play! demands more variance.

Optimization · Pure Optimization
A first-person puzzle game where seeing the solution is the whole skill.

Optimization · Pure Optimization
A tower-defense game mixing snowball action with scripted-wave strategy.
Portal 2 leans Optimization (Pure Optimization); South Park Let's Go Tower Defense Play! 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.
South Park Let's Go Tower Defense Play!
Snowball aiming adds light action demands
South Park Let's Go Tower Defense Play! leans a little harder on reads, though the gap is small.
Portal 2
No opponent; puzzles are static.
South Park Let's Go Tower Defense Play!
Enemy waves are scripted, no adaptive opponent
South Park Let's Go Tower Defense Play! leans clearly harder on variance.
Portal 2
Puzzles are deterministic with no randomness.
South Park Let's Go Tower Defense Play!
Wave order unknown first time, then fixed
Portal 2 leans clearly harder on optimization.
Portal 2
Solving the puzzle's logic IS the entire game.
South Park Let's Go Tower Defense Play!
Tower placement and resource management solve each map
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.