head to head
Neither is simply harder - they are hard in different places. Portal demands more execution; Scribblenauts demands more variance.

Optimization · Pure Optimization
The portal-gun puzzle game whose solutions are everything.

Optimization · Pure Optimization
Vocabulary-driven puzzle solving where the right summoned object cracks each level.
Portal leans Optimization (Pure Optimization); Scribblenauts leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Portal
Portal placement timing modest.
Scribblenauts
Stylus writing has no twitch demand
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Portal
No adversary modeling player.
Scribblenauts
Solo puzzles with no opponent to model
Scribblenauts leans clearly harder on variance.
Portal
Deterministic chambers; no randomness.
Scribblenauts
Puzzle setups are fixed once seen
Portal leans a little harder on optimization, though the gap is small.
Portal
Pure puzzle-solving optimization.
Scribblenauts
Finding the word that solves each puzzle IS the 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.