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

Optimization · Practiced Mastery
The 1986 original that defined exploration-driven, secret-riddled platforming.

Optimization · Practiced Mastery
Routing efficient digs and upgrades through a randomised mine.
Metroid leans Optimization (Practiced Mastery); SteamWorld Dig leans Optimization (Practiced Mastery).
Metroid leans a little harder on execution, though the gap is small.
Metroid
stiff NES jumps and tough enemy contact
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.
Metroid
creatures follow fixed movement
SteamWorld Dig
Enemies follow simple fixed patterns
SteamWorld Dig leans a little harder on variance, though the gap is small.
Metroid
unmarked secrets and bombable blocks hide the map
SteamWorld Dig
Randomised mine layouts hide ore and hazards each run
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Metroid
route and item knowledge transforms the game
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.