head to head
Neither is harder. They score identically on all four axes, so the difference between them is in kind, not degree.

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

Execution · Twitch Chaos
A classic kart racer balancing precise driving against heavily randomized item chaos.
Mario Kart 8 leans Execution (Twitch Chaos); Mario Kart Wii 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
Mario Kart Wii
drift timing and tricks require precision
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Mario Kart 8
minimal, mostly reacting not predicting
Mario Kart Wii
minimal opponent prediction, mostly reactive
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Mario Kart 8
item boxes are heavily randomized
Mario Kart Wii
blue shells and items are random
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Mario Kart 8
track lines and item strategy solvable
Mario Kart Wii
routes and item strategy are 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.