head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; SteamWorld Dig demands more variance.

Execution · Practiced Mastery
Sandbox crime playground spanning shooting, driving, and a persistent online economy.

Optimization · Practiced Mastery
Routing efficient digs and upgrades through a randomised mine.
Grand Theft Auto V leans Execution (Practiced Mastery); SteamWorld Dig leans Optimization (Practiced Mastery).
Grand Theft Auto V leans a little harder on execution, though the gap is small.
Grand Theft Auto V
Driving and shooting demand decent inputs.
SteamWorld Dig
Digging and platforming need modest precision
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
SteamWorld Dig
Enemies follow simple fixed patterns
SteamWorld Dig leans clearly harder on variance.
Grand Theft Auto V
Some mission and lobby randomness.
SteamWorld Dig
Randomised mine layouts hide ore and hazards each run
SteamWorld Dig leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
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.