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
Territory control through shooter execution and constant prediction of adapting human teams.

Reads · Competitive PvP
The Wii entry whose slower pace and infamous random tripping pushed Smash toward defensive mind games.
Splatoon leans Execution (Competitive PvP); Super Smash Bros. Brawl leans Reads (Competitive PvP).
Splatoon leans a little harder on execution, though the gap is small.
Splatoon
Aim, swim-form movement and ink management are constant demands
Super Smash Bros. Brawl
Slower engine lowers the ceiling but spacing still needs hands.
Super Smash Bros. Brawl leans a little harder on reads, though the gap is small.
Splatoon
Human opponents adapt flanks and pushes every match
Super Smash Bros. Brawl
Defensive bait-and-punish yomi dominates its floaty neutral.
Super Smash Bros. Brawl leans a little harder on variance, though the gap is small.
Splatoon
Hidden squids in ink create momentary information gaps
Super Smash Bros. Brawl
Random tripping injects genuine uncontrollable RNG into movement.
Splatoon leans a little harder on optimization, though the gap is small.
Splatoon
Weapon kits, map control timing and gear builds reward study
Super Smash Bros. Brawl
Matchup knowledge and ledge play reward study within a campy meta.
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.