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

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

Optimization · Pure Optimization
The puzzle-focused Game Boy Color Zelda built around hopping between two eras.
Portal 2 leans Optimization (Pure Optimization); The Legend of Zelda: Oracle of Ages 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.
The Legend of Zelda: Oracle of Ages
Modest combat demands by Zelda standards
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: Oracle of Ages
All enemies follow fixed patterns
The Legend of Zelda: Oracle of Ages leans clearly harder on variance.
Portal 2
Puzzles are deterministic with no randomness.
The Legend of Zelda: Oracle of Ages
Secrets hidden only until mapped
Portal 2 leans clearly harder on optimization.
Portal 2
Solving the puzzle's logic IS the entire game.
The Legend of Zelda: Oracle of Ages
Time-travel puzzles are the deliberate core test
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.