head to head
Virtua Tennis 4 asks more than Wii Sports, and the gap is execution.

Execution · Pure Mechanics
An arcade tennis game rewarding precise timing and reading human opponents.

Execution · Pure Mechanics
Motion-controlled tennis, bowling, golf, baseball and boxing built around simple swing gestures.
Virtua Tennis 4 leans Execution (Pure Mechanics); Wii Sports leans Execution (Pure Mechanics).
Virtua Tennis 4 leans clearly harder on execution.
Virtua Tennis 4
precise shot timing and positioning
Wii Sports
Motion swing timing and accuracy decide every event
Virtua Tennis 4 leans a little harder on reads, though the gap is small.
Virtua Tennis 4
human opponents can be out-predicted in rallies
Wii Sports
Casual local opponents add light mind games
Wii Sports leans a little harder on variance, though the gap is small.
Virtua Tennis 4
minor ball-bounce randomness
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.
Virtua Tennis 4
minimal build depth, mostly play skill
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.