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
Nine pointer-driven minigames bundled with a Wii Remote, from duck-hunt shooting to table tennis.

Execution · Competitive PvP
Arcade wrestling that tests reversal timing and little else by design.
Wii Play leans Execution (Pure Mechanics); WWE 2K Battlegrounds leans Execution (Competitive PvP).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Wii Play
Pointer aim and timing carry shooting and ping-pong games
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 Play
Two-player duels add slight mutual prediction
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 Play
Find Mii crowds and target spawns randomise lightly
WWE 2K Battlegrounds
Power-ups and interactive arenas inject party chaos
WWE 2K Battlegrounds leans a little harder on optimization, though the gap is small.
Wii Play
Minigames are too shallow to deeply optimise
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.