head to head
Neither is simply harder - they are hard in different places. Pong demands more reads; Wii Play demands more variance.

Execution · Competitive PvP
The original reflex duel - paddle precision with a thin layer of placement prediction.

Execution · Pure Mechanics
Nine pointer-driven minigames bundled with a Wii Remote, from duck-hunt shooting to table tennis.
Pong leans Execution (Competitive PvP); Wii Play leans Execution (Pure Mechanics).
Pong leans a little harder on execution, though the gap is small.
Pong
Paddle positioning and return angles are pure motor skill
Wii Play
Pointer aim and timing carry shooting and ping-pong games
Pong leans clearly harder on reads.
Pong
Against a human, anticipating placement adds a thin prediction layer
Wii Play
Two-player duels add slight mutual prediction
Wii Play leans clearly harder on variance.
Pong
Fully deterministic ball physics
Wii Play
Find Mii crowds and target spawns randomise lightly
Wii Play leans a little harder on optimization, though the gap is small.
Pong
Angle strategy is shallow and quickly exhausted
Wii Play
Minigames are too shallow to deeply optimise
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.