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
Motion-controlled racing where fighting the Kinect is half the physical test.

Execution · Pure Mechanics
Nine pointer-driven minigames bundled with a Wii Remote, from duck-hunt shooting to table tennis.
Sonic Free Riders leans Execution (Pure Mechanics); Wii Play leans Execution (Pure Mechanics).
Sonic Free Riders leans a little harder on execution, though the gap is small.
Sonic Free Riders
Full-body Kinect steering is a physical (if unreliable) motor test
Wii Play
Pointer aim and timing carry shooting and ping-pong games
Wii Play leans a little harder on reads, though the gap is small.
Sonic Free Riders
Human races add slight item-timing prediction
Wii Play
Two-player duels add slight mutual prediction
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Sonic Free Riders
Item pickups inject kart-style randomness
Wii Play
Find Mii crowds and target spawns randomise lightly
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Sonic Free Riders
Track shortcuts are shallow knowledge
Wii Play
Minigames are too shallow to deeply optimise
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.