head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; Super Castlevania IV demands more execution.

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

Execution · Practiced Mastery
Simon Belmont's SNES remake-quest with a multidirectional whip and a moody trek to Dracula.
Grand Theft Auto V leans Execution (Practiced Mastery); Super Castlevania IV leans Execution (Practiced Mastery).
Super Castlevania IV leans clearly harder on execution.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Super Castlevania IV
Eight-way whip control and stair commitment test precision
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Super Castlevania IV
Classic scripted monster patterns
Grand Theft Auto V leans a little harder on variance, though the gap is small.
Grand Theft Auto V
Some mission and lobby randomness.
Super Castlevania IV
Hidden wall meat, fixed locations
Super Castlevania IV leans a little harder on optimization, though the gap is small.
Grand Theft Auto V
Heists, money loops, builds learnable.
Super Castlevania IV
Subweapon choice and boss pattern study
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.