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

Optimization · Pure Optimization
A darker GameCube RPG where you snag corrupted Shadow Pokemon from enemy trainers in double battles.

Optimization · Pure Optimization
The portal-gun puzzle game whose solutions are everything.
Pokemon Colosseum leans Optimization (Pure Optimization); Portal leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Pokemon Colosseum
Turn-based double battles, menus only
Portal
Portal placement timing modest.
Pokemon Colosseum leans a little harder on reads, though the gap is small.
Pokemon Colosseum
All opponents scripted; no PvP focus
Portal
No adversary modeling player.
Pokemon Colosseum leans clearly harder on variance.
Pokemon Colosseum
Shadow Pokemon snag rolls and crits
Portal
Deterministic chambers; no randomness.
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Pokemon Colosseum
Double-battle synergy and purification planning dominate
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.