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 · Pure Mechanics
Keeping speed through destruction-racing chaos that punishes clean-lap plans.

Execution · Pure Mechanics
An action racer where players detonate the environment itself to wreck rivals and reroute the track.
FlatOut: Ultimate Carnage leans Execution (Pure Mechanics); Split/Second leans Execution (Pure Mechanics).
Split/Second leans a little harder on execution, though the gap is small.
FlatOut: Ultimate Carnage
Holding racing lines through wrecks and ragdoll chaos tests hands
Split/Second
Drafting, drifting and dodging collapsing set pieces.
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
FlatOut: Ultimate Carnage
Rival drivers ram predictably, light multiplayer jockeying
Split/Second
Timing powerplays against human rivals rewards prediction.
Split/Second leans a little harder on variance, though the gap is small.
FlatOut: Ultimate Carnage
Debris and pileups scramble any planned lap
Split/Second
Triggered explosions and route changes upend races constantly.
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
FlatOut: Ultimate Carnage
Car upgrades and track shortcuts are mappable
Split/Second
Knowing every trap and shortcut converges with play.
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.