head to head
Neither is harder. They score identically on all four axes, so the difference between them is in kind, not degree.

Optimization · Pure Optimization
Open-world detective work testing evidence deduction without hand-holding.

Optimization · Pure Optimization
A story-driven survival adventure alone on a hostile alien planet.
The Sinking City leans Optimization (Pure Optimization); The Solus Project leans Optimization (Pure Optimization).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
The Sinking City
Clunky combat is deliberately undemanding
The Solus Project
Exploration platforming with mild precision needs
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
The Sinking City
Monsters and citizens follow fixed scripts
The Solus Project
No adaptive entities, only environment
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
The Sinking City
Case clues are hidden information until investigated
The Solus Project
Alien caves hidden until explored, single playthrough mostly
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
The Sinking City
Cross-referencing evidence and city archives is the deduction core
The Solus Project
Managing temperature, hunger, and route planning drives survival
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.