head to head
Neither is simply harder - they are hard in different places. Planetbase demands more variance; The Elder Scrolls V: Skyrim demands more execution.

Optimization · Pure Optimization
Engineering self-sustaining closed-loop life support systems on a hostile planet against scripted environmental events.

Optimization · Pure Optimization
An open-world RPG mostly about discovering and stacking exploitable progression systems.
Planetbase leans Optimization (Pure Optimization); The Elder Scrolls V: Skyrim leans Optimization (Pure Optimization).
The Elder Scrolls V: Skyrim leans clearly harder on execution.
Planetbase
Building placement is unhurried and menu-driven throughout
The Elder Scrolls V: Skyrim
Combat is forgiving; aim and timing rarely punish you.
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Planetbase
No adversarial AI; threats are scripted event types
The Elder Scrolls V: Skyrim
Enemy AI is scripted and exploitable; not adaptive.
Planetbase leans clearly harder on variance.
Planetbase
Solar storm timing and resource availability add hidden pressure
The Elder Scrolls V: Skyrim
Loot tables and leveled lists inject mild randomness.
Planetbase leans a little harder on optimization, though the gap is small.
Planetbase
Power and oxygen loops are tightly engineered optimization puzzles
The Elder Scrolls V: Skyrim
Skill stacking, enchant loops, and exploits are deeply solvable.
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.