head to head
Neither is meaningfully harder - they pose close to the same test. The widest gap is optimization, and it is a single point.

Execution · Pure Mechanics
Punishing beat-em-up execution with zero margin and memorised stages.

Execution · Practiced Mastery
Simon Belmont's SNES remake-quest with a multidirectional whip and a moody trek to Dracula.
Double Dragon III: The Sacred Stones leans Execution (Pure Mechanics); Super Castlevania IV leans Execution (Practiced Mastery).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Double Dragon III: The Sacred Stones
Brutal difficulty and no continues punish every missed input
Super Castlevania IV
Eight-way whip control and stair commitment test precision
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Double Dragon III: The Sacred Stones
Enemies attack in fixed scripted patterns
Super Castlevania IV
Classic scripted monster patterns
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Double Dragon III: The Sacred Stones
Stages never change between attempts
Super Castlevania IV
Hidden wall meat, fixed locations
Super Castlevania IV leans a little harder on optimization, though the gap is small.
Double Dragon III: The Sacred Stones
Character-swap management and memorised layouts are the path through
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.