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 · Twitch Chaos
A kart racer where controlled chaos from random items tempers otherwise precise driving skill.

Execution · Twitch Chaos
A high-speed kart racer where team wisps and item variance sit atop a technique layer.
Mario Kart 8 leans Execution (Twitch Chaos); Sonic Racing CrossWorlds leans Execution (Twitch Chaos).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Mario Kart 8
precise drifting and item timing matter
Sonic Racing CrossWorlds
Speed-type technique and boost activation have a real ceiling
Sonic Racing CrossWorlds leans a little harder on reads, though the gap is small.
Mario Kart 8
minimal, mostly reacting not predicting
Sonic Racing CrossWorlds
Online races require light item-anticipation modelling
Mario Kart 8 leans a little harder on variance, though the gap is small.
Mario Kart 8
item boxes are heavily randomized
Sonic Racing CrossWorlds
Wisp items and random pickups generate high per-race variance
Mario Kart 8 leans a little harder on optimization, though the gap is small.
Mario Kart 8
track lines and item strategy solvable
Sonic Racing CrossWorlds
Team ability combos and track shortcuts are learnable solvable systems
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.