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
Fast mech duels testing piloting execution and prediction of human opponents.

Execution · Competitive PvP
Territory control through shooter execution and constant prediction of adapting human teams.
Mecha Break leans Execution (Competitive PvP); Splatoon 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.
Mecha Break
Boost management and aim in fast mech duels demand dexterity
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.
Mecha Break
Human pilots adapt positioning and engage timing constantly
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.
Mecha Break
Match conditions are mostly fixed beyond extraction-mode loot
Splatoon
Hidden squids in ink create momentary information gaps
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Mecha Break
Mech loadouts and role matchups reward roster 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.