head to head
Neither is simply harder - they are hard in different places. Dead Rising 3 demands more optimization; Grand Theft Auto V demands more reads.

Optimization · Practiced Mastery
Crafting absurd weapons and routing a zombie-filled city against the clock.

Execution · Practiced Mastery
Sandbox crime playground spanning shooting, driving, and a persistent online economy.
Dead Rising 3 leans Optimization (Practiced Mastery); Grand Theft Auto V leans Execution (Practiced Mastery).
Grand Theft Auto V leans a little harder on execution, though the gap is small.
Dead Rising 3
Crowd fighting takes modest aim and dodge timing
Grand Theft Auto V
Driving and shooting demand decent inputs.
Grand Theft Auto V leans clearly harder on reads.
Dead Rising 3
Zombies and psychopaths run fixed scripts, never adapt
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Dead Rising 3
Weapon and blueprint locations are unknown until explored
Grand Theft Auto V
Some mission and lobby randomness.
Dead Rising 3 leans clearly harder on optimization.
Dead Rising 3
Combo weapon crafting and timed routing drive efficient runs
Grand Theft Auto V
Heists, money loops, builds learnable.
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.