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 · Pure Mechanics
Multiplayer-focused Halo with the series' highest movement skill ceiling.

Execution · Practiced Mastery
A team shooter where aim and positioning against human opponents controls the map.
Halo 5: Guardians leans Execution (Pure Mechanics); Splatoon 2 leans Execution (Practiced Mastery).
Halo 5: Guardians leans a little harder on execution, though the gap is small.
Halo 5: Guardians
Thrusters, clamber, and aim raise the ceiling
Splatoon 2
aim and movement decide ink battles
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Halo 5: Guardians
Arena ranked play is human-versus-human
Splatoon 2
predicting human opponent positioning
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Halo 5: Guardians
Spawns and REQ Warzone randomness
Splatoon 2
weapon and map rotation randomness
Splatoon 2 leans a little harder on optimization, though the gap is small.
Halo 5: Guardians
Movement routes and weapon timers
Splatoon 2
loadout and turf strategy 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.