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.

Optimization · Pure Optimization
An infamously cryptic NES adventure where lying villagers obscure how to break Dracula's curse.

Optimization · Practiced Mastery
Routing efficient digs and upgrades through a randomised mine.
Castlevania II: Simon's Quest leans Optimization (Pure Optimization); SteamWorld Dig leans Optimization (Practiced Mastery).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Castlevania II: Simon's Quest
Modest whip combat; day-night enemies.
SteamWorld Dig
Digging and platforming need modest precision
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Castlevania II: Simon's Quest
No adaptive opposition.
SteamWorld Dig
Enemies follow simple fixed patterns
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Castlevania II: Simon's Quest
Deliberately false villager clues hide the real path.
SteamWorld Dig
Randomised mine layouts hide ore and hazards each run
Castlevania II: Simon's Quest leans a little harder on optimization, though the gap is small.
Castlevania II: Simon's Quest
Once decoded, it is pure item-and-route knowledge.
SteamWorld Dig
Upgrade routing and efficient dig paths drive progress
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.