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

Execution · Pure Mechanics
The hardest NES Castlevania, with branching routes and switchable partner characters.

Execution · Pure Mechanics
A boss-rush action game that is nothing but execution tests against scripted duelists.
Castlevania III: Dracula's Curse leans Execution (Pure Mechanics); Furi leans Execution (Pure Mechanics).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Castlevania III: Dracula's Curse
Brutal platforming and tight whip timing throughout
Furi
Parry timing, bullet dodging, and duel precision
Furi leans a little harder on reads, though the gap is small.
Castlevania III: Dracula's Curse
Scripted enemies and bosses only
Furi
Bosses are scripted patterns, however adversarial they feel
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Castlevania III: Dracula's Curse
Minor enemy spawn wobble
Furi
Fixed boss sequences
Castlevania III: Dracula's Curse leans a little harder on optimization, though the gap is small.
Castlevania III: Dracula's Curse
Partner choice, route branching, pattern memory
Furi
Learning each boss's phases converges to mastery
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.