head to head
Neither is simply harder - they are hard in different places. Medieval II: Total War demands more variance; Portal demands more execution.

Optimization · Pure Optimization
Grand strategy wargame mixing turn-based empire play with real-time battles.

Optimization · Pure Optimization
The portal-gun puzzle game whose solutions are everything.
Medieval II: Total War leans Optimization (Pure Optimization); Portal leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Medieval II: Total War
real-time battle micro is modest
Portal
Portal placement timing modest.
Medieval II: Total War leans a little harder on reads, though the gap is small.
Medieval II: Total War
campaign AI is exploitable and mappable
Portal
No adversary modeling player.
Medieval II: Total War leans clearly harder on variance.
Medieval II: Total War
fog of war and event triggers hide some state
Portal
Deterministic chambers; no randomness.
Medieval II: Total War leans a little harder on optimization, though the gap is small.
Medieval II: Total War
economy, unit counters, and campaign map strategy 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.