head to head
Neither is simply harder - they are hard in different places. Populous: The Beginning demands more reads; Portal 2 demands more optimization.

Optimization · Pure Optimization
RTS-flavoured god game where a shaman terraforms planets and rains fire on rivals.

Optimization · Pure Optimization
A first-person puzzle game where seeing the solution is the whole skill.
Populous: The Beginning leans Optimization (Pure Optimization); Portal 2 leans Optimization (Pure Optimization).
Populous: The Beginning leans a little harder on execution, though the gap is small.
Populous: The Beginning
Spell timing and unit micro demand real-time hands.
Portal 2
Portal placement requires some precision but timing is lenient.
Populous: The Beginning leans clearly harder on reads.
Populous: The Beginning
Multiplayer shaman duels involve some opponent prediction.
Portal 2
No opponent; puzzles are static.
Populous: The Beginning leans clearly harder on variance.
Populous: The Beginning
Unscouted terrain and totems hide briefly.
Portal 2
Puzzles are deterministic with no randomness.
Portal 2 leans clearly harder on optimization.
Populous: The Beginning
Mana ramp, building order and spell economy are solvable.
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.