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 and reversal timing with light mindgames against human opponents.
Wii Sports leans Execution (Pure Mechanics); WWF SmackDown! Just Bring It 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 SmackDown! Just Bring It
Grapple timing and reversal windows decide matches
WWF SmackDown! Just Bring It leans a little harder on reads, though the gap is small.
Wii Sports
Casual local opponents add light mind games
WWF SmackDown! Just Bring It
Human opponents invite light reversal mindgames
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 SmackDown! Just Bring It
Weapon spawns and AI interference add small chaos
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 SmackDown! Just Bring It
Move-set knowledge and match-type tactics are quickly learned
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.