head to head
Neither is simply harder - they are hard in different places. Harvest Moon: Friends of Mineral Town demands more variance; Portal demands more execution.

Optimization · Pure Optimization
Optimising daily routines, crop economics and relationship schedules on a fixed calendar.

Optimization · Pure Optimization
The portal-gun puzzle game whose solutions are everything.
Harvest Moon: Friends of Mineral Town leans Optimization (Pure Optimization); Portal leans Optimization (Pure Optimization).
Portal leans clearly harder on execution.
Harvest Moon: Friends of Mineral Town
Simple tool taps - stamina, not dexterity, limits you
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.
Harvest Moon: Friends of Mineral Town
Villagers follow fixed schedules, nothing adapts
Portal
No adversary modeling player.
Harvest Moon: Friends of Mineral Town leans clearly harder on variance.
Harvest Moon: Friends of Mineral Town
Weather and festival rolls add mild noise
Portal
Deterministic chambers; no randomness.
Portal leans a little harder on optimization, though the gap is small.
Harvest Moon: Friends of Mineral Town
Crop math, daily routing and gift schedules are fully solvable
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.