head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto: San Andreas demands more optimization; Zombie Army Trilogy demands more execution.

Optimization · Practiced Mastery
A sprawling open-world crime epic with RPG-style character stats across a whole state.

Execution · Practiced Mastery
A co-op horde shooter testing sniper aim and chokepoint planning against scripted waves.
Grand Theft Auto: San Andreas leans Optimization (Practiced Mastery); Zombie Army Trilogy leans Execution (Practiced Mastery).
Zombie Army Trilogy leans clearly harder on execution.
Grand Theft Auto: San Andreas
Flying school and drive-bys need decent hands
Zombie Army Trilogy
Long-range sniping and horde kiting demand steady aim
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Grand Theft Auto: San Andreas
Gang AI is scripted, never adaptive
Zombie Army Trilogy
Undead waves follow predictable spawn scripts
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Grand Theft Auto: San Andreas
Territory wars and traffic add light randomness
Zombie Army Trilogy
Spawn timing varies a little between runs
Grand Theft Auto: San Andreas leans clearly harder on optimization.
Grand Theft Auto: San Andreas
Stats, gym, weapon skill, and routes reward systematic play
Zombie Army Trilogy
Choke points, trap placement and ammo economy reward planning
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.