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
Co-op zombie shooter whose AI Director reshuffles every run to keep teams honest.

Execution · Twitch Chaos
Kart racing where driving skill fights a steady drizzle of random gag items.
Left 4 Dead leans Execution (Twitch Chaos); Looney Tunes: Space Race leans Execution (Twitch Chaos).
Left 4 Dead leans a little harder on execution, though the gap is small.
Left 4 Dead
Headshots, melee shoves, and special-infected reaction time
Looney Tunes: Space Race
Racing lines and drift control still decide most finishes
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Left 4 Dead
Versus mode adds human ambush mindgames
Looney Tunes: Space Race
Light gag-timing duels against human racers
Left 4 Dead leans a little harder on variance, though the gap is small.
Left 4 Dead
AI Director randomizes hordes, items, and specials
Looney Tunes: Space Race
Random gag pickups scramble race outcomes
Left 4 Dead leans a little harder on optimization, though the gap is small.
Left 4 Dead
Team spacing, route choice, and resource discipline
Looney Tunes: Space Race
Shortcut and track knowledge compound every lap
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.