head to head
Neither is simply harder - they are hard in different places. Contagion demands more reads; Mario Kart 8 demands more variance.

Execution · Twitch Chaos
A scarcity-driven aim test under randomised objectives and spawns.

Execution · Twitch Chaos
A kart racer where controlled chaos from random items tempers otherwise precise driving skill.
Contagion leans Execution (Twitch Chaos); Mario Kart 8 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.
Contagion
Deliberate aim with scarce ammo decides zombie encounters
Mario Kart 8
precise drifting and item timing matter
Contagion leans clearly harder on reads.
Contagion
Hunted mode pits humans against humans; zombies are mindless
Mario Kart 8
minimal, mostly reacting not predicting
Mario Kart 8 leans clearly harder on variance.
Contagion
Randomised objectives, spawns and routes reshape every round
Mario Kart 8
item boxes are heavily randomized
Mario Kart 8 leans a little harder on optimization, though the gap is small.
Contagion
Resource discipline and map knowledge carry survival
Mario Kart 8
track lines and item 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.