head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; Ratchet & Clank: Into the Nexus demands more optimization.

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

Execution · Practiced Mastery
A third-person action platformer where you blast scripted alien hordes while leveling an arsenal of gadgets.
Grand Theft Auto V leans Execution (Practiced Mastery); Ratchet & Clank: Into the Nexus leans Execution (Practiced Mastery).
Ratchet & Clank: Into the Nexus leans a little harder on execution, though the gap is small.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Ratchet & Clank: Into the Nexus
Aiming, strafing, and platforming under fire carry combat
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Ratchet & Clank: Into the Nexus
Scripted enemy waves; no adaptive opponent
Grand Theft Auto V leans a little harder on variance, though the gap is small.
Grand Theft Auto V
Some mission and lobby randomness.
Ratchet & Clank: Into the Nexus
Fixed level layouts, minimal hidden information
Ratchet & Clank: Into the Nexus leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
Ratchet & Clank: Into the Nexus
Weapon upgrade paths, bolt economy, gold bolt routing
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.