head to head
Neither is simply harder - they are hard in different places. Etrian Odyssey Untold: The Millennium Girl demands more variance; Portal 2 demands more execution.

Optimization · Pure Optimization
Mapping dungeons and engineering party builds against chartable threats.

Optimization · Pure Optimization
A first-person puzzle game where seeing the solution is the whole skill.
Etrian Odyssey Untold: The Millennium Girl leans Optimization (Pure Optimization); Portal 2 leans Optimization (Pure Optimization).
Portal 2 leans clearly harder on execution.
Etrian Odyssey Untold: The Millennium Girl
Turn-based menus require no dexterity
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.
Etrian Odyssey Untold: The Millennium Girl
FOE patterns are scripted and chartable
Portal 2
No opponent; puzzles are static.
Etrian Odyssey Untold: The Millennium Girl leans clearly harder on variance.
Etrian Odyssey Untold: The Millennium Girl
Unknown floors and ambushes punish until mapped
Portal 2
Puzzles are deterministic with no randomness.
Portal 2 leans a little harder on optimization, though the gap is small.
Etrian Odyssey Untold: The Millennium Girl
Party builds, skill synergies and map routing dominate
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.