head to head
Neither is meaningfully harder - they pose close to the same test. The widest gap is execution, and it is a single point.

Execution · Twitch Chaos
A kart racer with random items and drift timing on themed circuits.

Execution · Twitch Chaos
Co-op zombie shooter whose AI Director reshuffles every run to keep teams honest.
Cartoon Network Racing leans Execution (Twitch Chaos); Left 4 Dead leans Execution (Twitch Chaos).
Left 4 Dead leans a little harder on execution, though the gap is small.
Cartoon Network Racing
Steering, drifting and item timing decide races
Left 4 Dead
Headshots, melee shoves, and special-infected reaction time
Left 4 Dead leans a little harder on reads, though the gap is small.
Cartoon Network Racing
AI racers scripted, no adaptive prediction
Left 4 Dead
Versus mode adds human ambush mindgames
Left 4 Dead leans a little harder on variance, though the gap is small.
Cartoon Network Racing
Random item pickups swing race outcomes
Left 4 Dead
AI Director randomizes hordes, items, and specials
Left 4 Dead leans a little harder on optimization, though the gap is small.
Cartoon Network Racing
Shortcuts and item use reward track knowledge
Left 4 Dead
Team spacing, route choice, and resource discipline
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.