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
Trilogy capstone whose equal-start arena multiplayer defined a console generation.

Execution · Competitive PvP
A class-based shooter spin-off of the tower-defense series with PvP and co-op modes.
Halo 3 leans Execution (Pure Mechanics); Plants vs. Zombies: Battle for Neighborville leans Execution (Competitive PvP).
Halo 3 leans a little harder on execution, though the gap is small.
Halo 3
Precision aim and grenade-melee timing
Plants vs. Zombies: Battle for Neighborville
aim and movement in class-based shooter combat
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Halo 3
Equal-start fights reward predicting humans
Plants vs. Zombies: Battle for Neighborville
predicting human opponents' pushes and flanks
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Halo 3
Equipment pickups and spawn variation
Plants vs. Zombies: Battle for Neighborville
minor loot and spawn randomness
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Halo 3
Power-weapon timers and map control
Plants vs. Zombies: Battle for Neighborville
class builds and lane strategy are solvable
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.