head to head
Neither is meaningfully harder - they pose close to the same test. The widest gap is execution, and it is a single point.

Execution · Practiced Mastery
The fastest mainline Souls, distilling the formula into aggressive boss duels.

Execution · Practiced Mastery
A rhythm-parry action game about reading scripted tells with frame-tight timing.
Dark Souls III leans Execution (Practiced Mastery); Sekiro: Shadows Die Twice leans Execution (Practiced Mastery).
Sekiro: Shadows Die Twice leans a little harder on execution, though the gap is small.
Dark Souls III
fast roll timing against aggressive scripted bosses
Sekiro: Shadows Die Twice
Frame-perfect parries are the entire combat system.
Sekiro: Shadows Die Twice leans a little harder on reads, though the gap is small.
Dark Souls III
PvP invasions exist but are optional
Sekiro: Shadows Die Twice
Mikiri and perilous tells are scripted, not adaptive.
Dark Souls III leans a little harder on variance, though the gap is small.
Dark Souls III
fixed encounters converge after first sight
Sekiro: Shadows Die Twice
Almost no randomness; encounters are deterministic.
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Dark Souls III
movesets and builds are fully mappable
Sekiro: Shadows Die Twice
Boss decks and prosthetic choices are fully learnable.
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.