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 · Practiced Mastery
343's debut pits Master Chief against the Didact with loadout-driven multiplayer.

Execution · Practiced Mastery
A team shooter where aim and positioning against human opponents controls the map.
Halo 4 leans Execution (Practiced Mastery); Splatoon 2 leans Execution (Practiced Mastery).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Halo 4
Precision aim amid faster kill times
Splatoon 2
aim and movement decide ink battles
Splatoon 2 leans a little harder on reads, though the gap is small.
Halo 4
Human opponents in Infinity multiplayer
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 4
Ordnance drops randomise mid-match rewards
Splatoon 2
weapon and map rotation randomness
Splatoon 2 leans a little harder on optimization, though the gap is small.
Halo 4
Loadout building and map control
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.