head to head
Neither is simply harder - they are hard in different places. Portal 2 demands more optimization; The Legend of Zelda: Spirit Tracks demands more variance.

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

Optimization · Pure Optimization
A touch-controlled DS Zelda about puzzling through dungeons and driving a train across Hyrule.
Portal 2 leans Optimization (Pure Optimization); The Legend of Zelda: Spirit Tracks leans Optimization (Pure Optimization).
The Legend of Zelda: Spirit Tracks leans a little harder on execution, though the gap is small.
Portal 2
Portal placement requires some precision but timing is lenient.
The Legend of Zelda: Spirit Tracks
Stylus combat and flute playing are forgiving
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Portal 2
No opponent; puzzles are static.
The Legend of Zelda: Spirit Tracks
Phantoms patrol on fixed rules
The Legend of Zelda: Spirit Tracks leans clearly harder on variance.
Portal 2
Puzzles are deterministic with no randomness.
The Legend of Zelda: Spirit Tracks
Unknown layouts only until explored
Portal 2 leans clearly harder on optimization.
Portal 2
Solving the puzzle's logic IS the entire game.
The Legend of Zelda: Spirit Tracks
Rail routing and tower puzzles are the core
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.