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

Execution · Competitive PvP
Tests stylish movement-shooter mechanics against human teams in objective modes.

Execution · Competitive PvP
Territory control through shooter execution and constant prediction of adapting human teams.
S4 League 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.
S4 League
Wall-running movement and aim duels decide exchanges
Splatoon
Aim, swim-form movement and ink management are constant demands
Splatoon leans a little harder on reads, though the gap is small.
S4 League
Human opponents in Touchdown demand constant prediction
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.
S4 League
Some weapon spread and mode chaos
Splatoon
Hidden squids in ink create momentary information gaps
Splatoon leans a little harder on optimization, though the gap is small.
S4 League
Loadout choices and map knowledge matter underneath
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.