head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; ZombiU demands more variance.

Execution · Practiced Mastery
Sandbox crime playground spanning shooting, driving, and a persistent online economy.

Optimization · Practiced Mastery
A tense survival horror where one bite kills your survivor for good.
Grand Theft Auto V leans Execution (Practiced Mastery); ZombiU leans Optimization (Practiced Mastery).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Grand Theft Auto V
Driving and shooting demand decent inputs.
ZombiU
Deliberate cricket-bat timing and panic inventory management
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
ZombiU
Zombies shamble on fixed logic
ZombiU leans clearly harder on variance.
Grand Theft Auto V
Some mission and lobby randomness.
ZombiU
Permadeath runs face unknown spawn placements
ZombiU leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
ZombiU
Route knowledge, supply caching, safehouse 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.