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 · Competitive PvP
A flashy arena fighter testing reaction timing and substitution mind-games against humans.

Execution · Competitive PvP
Territory control through shooter execution and constant prediction of adapting human teams.
Naruto x Boruto: Ultimate Ninja Storm Connections leans Execution (Competitive PvP); Splatoon leans Execution (Competitive PvP).
Splatoon leans a little harder on execution, though the gap is small.
Naruto x Boruto: Ultimate Ninja Storm Connections
Combo timing and substitution reactions decide exchanges
Splatoon
Aim, swim-form movement and ink management are constant demands
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Naruto x Boruto: Ultimate Ninja Storm Connections
Human opponents bait substitutions and punish habits
Splatoon
Human opponents adapt flanks and pushes every match
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Naruto x Boruto: Ultimate Ninja Storm Connections
Little hidden beyond occasional support-assist surprises
Splatoon
Hidden squids in ink create momentary information gaps
Splatoon leans a little harder on optimization, though the gap is small.
Naruto x Boruto: Ultimate Ninja Storm Connections
Team composition and meter management reward system knowledge
Splatoon
Weapon kits, map control timing and gear builds reward study
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.