head to head
Neither is harder. They score identically on all four axes, so the difference between them is in kind, not degree.

Execution · Practiced Mastery
The Genesis Castlevania tours war-torn Europe with whip-wielding John Morris and spear-vaulting Eric Lecarde.

Execution · Practiced Mastery
The fan-favourite sequel with harder levels and deeper secrets.
Castlevania: Bloodlines leans Execution (Practiced Mastery); Donkey Kong Country 2: Diddy's Kong Quest 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.
Castlevania: Bloodlines
Stiff classic movement against demanding Genesis bosses
Donkey Kong Country 2: Diddy's Kong Quest
Demanding jump chains and animal-buddy control
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Castlevania: Bloodlines
Scripted monsters throughout
Donkey Kong Country 2: Diddy's Kong Quest
Fixed enemy patterns, no adaptation
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Castlevania: Bloodlines
Minor hidden items only
Donkey Kong Country 2: Diddy's Kong Quest
Secret Lost World entrances hidden until found
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Castlevania: Bloodlines
Character choice changes paths; patterns are learnable
Donkey Kong Country 2: Diddy's Kong Quest
Routes, DK coins, and bonuses fully mappable
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.