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

Execution · Competitive PvP
Class-based team shooter with deep mind games.
Splatoon leans Execution (Competitive PvP); Team Fortress 2 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
Team Fortress 2
Projectile aim and rocket jumps reward hands
Team Fortress 2 leans a little harder on reads, though the gap is small.
Splatoon
Human opponents adapt flanks and pushes every match
Team Fortress 2
Spy/sniper duels are pure prediction
Team Fortress 2 leans a little harder on variance, though the gap is small.
Splatoon
Hidden squids in ink create momentary information gaps
Team Fortress 2
Random crits and spread inject RNG
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
Team Fortress 2
Class loadouts and map routes solvable
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.