head to head
Neither is harder. They score identically on all four axes, so the difference between them is in kind, not degree.

Optimization · Pure Optimization
Tower-defense math wrapped in a city-building economy to optimise.

Optimization · Pure Optimization
Lane defense classic about managing a sun economy against shambling waves.
Bloons Monkey City leans Optimization (Pure Optimization); Plants vs. Zombies leans Optimization (Pure Optimization).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Bloons Monkey City
Tower placement is unhurried clicking
Plants vs. Zombies
Clicking to plant, minimal pressure
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Bloons Monkey City
Bloon waves never adapt to your defenses
Plants vs. Zombies
Zombie waves follow set compositions
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Bloons Monkey City
Captured tile layouts and special events vary mildly
Plants vs. Zombies
Wave timing and lane pressure vary mildly
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Bloons Monkey City
Tower math, city economy and unlock routing are the whole game
Plants vs. Zombies
Sun economy and lane setups are near-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.