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
Motion-controlled tennis, bowling, golf, baseball and boxing built around simple swing gestures.

Execution · Pure Mechanics
Grapple-timing execution and mashing stamina in arcade wrestling matches.
Wii Sports leans Execution (Pure Mechanics); WWF Royal Rumble 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.
Wii Sports
Motion swing timing and accuracy decide every event
WWF Royal Rumble
Grapple timing and button mashing decide every lock-up
WWF Royal Rumble leans a little harder on reads, though the gap is small.
Wii Sports
Casual local opponents add light mind games
WWF Royal Rumble
Human opponents add some habit prediction in versus
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Wii Sports
Minor physics wobble and CPU behaviour randomness
WWF Royal Rumble
Little is hidden beyond grapple outcome wobble
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Wii Sports
Simple technique tricks like bowling spin are learnable
WWF Royal Rumble
Move utility knowledge is a shallow solvable layer
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.