head to head
Mario Kart 8 asks more than NASCAR Rumble, and the gap is variance.

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

Execution · Twitch Chaos
Arcade stock-car driving where random power-ups keep races scrambled.
Mario Kart 8 leans Execution (Twitch Chaos); NASCAR Rumble 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
NASCAR Rumble
Arcade handling at speed still rewards clean driving
NASCAR Rumble leans a little harder on reads, though the gap is small.
Mario Kart 8
minimal, mostly reacting not predicting
NASCAR Rumble
Split-screen rivals can be baited with items
Mario Kart 8 leans clearly harder on variance.
Mario Kart 8
item boxes are heavily randomized
NASCAR Rumble
Power-up pickups scramble race outcomes constantly
Mario Kart 8 leans clearly harder on optimization.
Mario Kart 8
track lines and item strategy solvable
NASCAR Rumble
Track shortcuts and item timing are learnable edges
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.