head to head
Neither is simply harder - they are hard in different places. Mario Kart World demands more reads; Split/Second demands more execution.

Execution · Twitch Chaos
Kart racing blending drift execution with item RNG and pack positioning.

Execution · Pure Mechanics
An action racer where players detonate the environment itself to wreck rivals and reroute the track.
Mario Kart World leans Execution (Twitch Chaos); Split/Second leans Execution (Pure Mechanics).
Split/Second leans clearly harder on execution.
Mario Kart World
Drift charging, rail grinding and shortcut precision reward hands
Split/Second
Drafting, drifting and dodging collapsing set pieces.
Mario Kart World leans clearly harder on reads.
Mario Kart World
Human racers bait items and contest lines in knockouts
Split/Second
Timing powerplays against human rivals rewards prediction.
Mario Kart World leans clearly harder on variance.
Mario Kart World
Item rolls and rubber-banding scramble any single race
Split/Second
Triggered explosions and route changes upend races constantly.
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Mario Kart World
Course knowledge, shortcuts and item management converge with study
Split/Second
Knowing every trap and shortcut converges with play.
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.