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
Stick precision and constant prediction of a human opponent's tactical habits.

Execution · Competitive PvP
Territory control through shooter execution and constant prediction of adapting human teams.
Pro Evolution Soccer 2015 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.
Pro Evolution Soccer 2015
First touch, through-ball timing and finishing demand precise sticks
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.
Pro Evolution Soccer 2015
Human opponents bait, switch tactics 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.
Pro Evolution Soccer 2015
Deflections and referee calls add bounded chaos
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.
Pro Evolution Soccer 2015
Formations, player roles and set pieces reward study
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.