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

Execution · Competitive PvP
Arcade wrestling that tests reversal timing and little else by design.
Wii Sports leans Execution (Pure Mechanics); WWE 2K Battlegrounds leans Execution (Competitive PvP).
Wii Sports leans a little harder on execution, though the gap is small.
Wii Sports
Motion swing timing and accuracy decide every event
WWE 2K Battlegrounds
Button-mash combos and reversal timing set the pace
WWE 2K Battlegrounds leans a little harder on reads, though the gap is small.
Wii Sports
Casual local opponents add light mind games
WWE 2K Battlegrounds
Human opponents add light reversal mind games
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
WWE 2K Battlegrounds
Power-ups and interactive arenas inject party 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
WWE 2K Battlegrounds
Shallow systems - move knowledge helps a little
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.