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

Execution · Competitive PvP
Territory control through shooter execution and constant prediction of adapting human teams.

Execution · Competitive PvP
Tests stick skills and constant prediction of a human opponent's attacking intent.
Splatoon leans Execution (Competitive PvP); World Soccer Winning Eleven 6: Final Evolution leans Execution (Competitive PvP).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Splatoon
Aim, swim-form movement and ink management are constant demands
World Soccer Winning Eleven 6: Final Evolution
First-touch timing, through-balls and finishing are mechanical skill
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Splatoon
Human opponents adapt flanks and pushes every match
World Soccer Winning Eleven 6: Final Evolution
Human opponents constantly disguise attacks and adjust defending
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Splatoon
Hidden squids in ink create momentary information gaps
World Soccer Winning Eleven 6: Final Evolution
Deflections and keeper bounces add minor chaos
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Splatoon
Weapon kits, map control timing and gear builds reward study
World Soccer Winning Eleven 6: Final Evolution
Formations, Master League squad building reward system knowledge
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.