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

Execution · Pure Mechanics
Arcade soccer testing reflex tackles and super-shot timing.

Execution · Pure Mechanics
Motion-controlled tennis, bowling, golf, baseball and boxing built around simple swing gestures.
Nintendo World Cup leans Execution (Pure Mechanics); Wii Sports leans Execution (Pure Mechanics).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Nintendo World Cup
Tackles and super-shot timing decide most goals
Wii Sports
Motion swing timing and accuracy decide every event
Nintendo World Cup leans a little harder on reads, though the gap is small.
Nintendo World Cup
Versus a human, shot fakes add light prediction
Wii Sports
Casual local opponents add light mind games
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Nintendo World Cup
Ball physics add minor chaos
Wii Sports
Minor physics wobble and CPU behaviour randomness
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Nintendo World Cup
Learning super-move triggers is a small solvable layer
Wii Sports
Simple technique tricks like bowling spin 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.