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
A classic maze-building tower defense won almost entirely through optimal layouts.

Optimization · Pure Optimization
A knowledge-based mystery solved by mapping a deterministic solar system.
Defense Grid 2 leans Optimization (Pure Optimization); Outer Wilds 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.
Defense Grid 2
Occasional orbital-ability timing, otherwise hands-off
Outer Wilds
Light piloting, mostly forgiving
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Defense Grid 2
Fixed alien wave scripts, nothing adapts
Outer Wilds
World is fixed; nothing adapts to you
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Defense Grid 2
Spawn schedules are deterministic and previewable
Outer Wilds
Loop is deterministic once understood
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Defense Grid 2
Maze routing and tower economy are a solvable puzzle
Outer Wilds
Pure knowledge puzzle - solve the system
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.