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

Optimization · Pure Optimization
Tower-defense loadout math wearing an Adventure Time crossover costume.

Optimization · Pure Optimization
The portal-gun puzzle game whose solutions are everything.
Bloons Adventure Time TD leans Optimization (Pure Optimization); Portal leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Bloons Adventure Time TD
Tower placement and occasional ability taps
Portal
Portal placement timing modest.
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Bloons Adventure Time TD
Bloon rushes follow fixed wave scripts
Portal
No adversary modeling player.
Bloons Adventure Time TD leans clearly harder on variance.
Bloons Adventure Time TD
Character trinket drops add light collection RNG
Portal
Deterministic chambers; no randomness.
Portal leans a little harder on optimization, though the gap is small.
Bloons Adventure Time TD
Hero, allies and trinket loadout math drives every map
Portal
Pure puzzle-solving optimization.
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.