head to head
Neither is simply harder - they are hard in different places. Neighbours from Hell demands more variance; Portal 2 demands more optimization.

Optimization · Pure Optimization
Sitcom puzzle game about rigging perfect prank chains around a fixed routine.

Optimization · Pure Optimization
A first-person puzzle game where seeing the solution is the whole skill.
Neighbours from Hell leans Optimization (Pure Optimization); Portal 2 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.
Neighbours from Hell
Timing pranks between patrol windows
Portal 2
Portal placement requires some precision but timing is lenient.
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Neighbours from Hell
The neighbour loops a fixed routine
Portal 2
No opponent; puzzles are static.
Neighbours from Hell leans clearly harder on variance.
Neighbours from Hell
Routine unknown on first attempt, then converges
Portal 2
Puzzles are deterministic with no randomness.
Portal 2 leans clearly harder on optimization.
Neighbours from Hell
Sequencing pranks for maximum combo rating
Portal 2
Solving the puzzle's logic IS the entire game.
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.