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
GameCube kart racer with two riders per kart trading items mid-drift.

Execution · Twitch Chaos
Kart racing execution moderated by item randomness and learnable track routes.
Mario Kart: Double Dash!! leans Execution (Twitch Chaos); Sonic & All-Stars Racing Transformed leans Execution (Twitch Chaos).
Mario Kart: Double Dash!! leans a little harder on execution, though the gap is small.
Mario Kart: Double Dash!!
Two-character swaps and drift timing
Sonic & All-Stars Racing Transformed
Drift timing and transforming vehicle control reward practice
Mario Kart: Double Dash!! leans a little harder on reads, though the gap is small.
Mario Kart: Double Dash!!
Item duels against humans reward baiting
Sonic & All-Stars Racing Transformed
Human racers add light positional mind games
Mario Kart: Double Dash!! leans a little harder on variance, though the gap is small.
Mario Kart: Double Dash!!
Special item rolls swing races
Sonic & All-Stars Racing Transformed
Item rolls regularly upend race positions
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Mario Kart: Double Dash!!
Character pairings and lines are solved
Sonic & All-Stars Racing Transformed
Track shortcuts and drift-boost routes are learnable
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.