head to head
Neither is simply harder - they are hard in different places. Diddy Kong Racing DS demands more execution; Grand Theft Auto: San Andreas demands more optimization.

Execution · Practiced Mastery
Tests kart-racing line execution across three vehicle types with item luck on top.

Optimization · Practiced Mastery
A sprawling open-world crime epic with RPG-style character stats across a whole state.
Diddy Kong Racing DS leans Execution (Practiced Mastery); Grand Theft Auto: San Andreas leans Optimization (Practiced Mastery).
Diddy Kong Racing DS leans clearly harder on execution.
Diddy Kong Racing DS
Drift boosting across car, hover and plane handling
Grand Theft Auto: San Andreas
Flying school and drive-bys need decent hands
Diddy Kong Racing DS leans a little harder on reads, though the gap is small.
Diddy Kong Racing DS
AI rivals are scripted; multiplayer prediction stays shallow
Grand Theft Auto: San Andreas
Gang AI is scripted, never adaptive
Diddy Kong Racing DS leans a little harder on variance, though the gap is small.
Diddy Kong Racing DS
Balloon item pickups swing race positions
Grand Theft Auto: San Andreas
Territory wars and traffic add light randomness
Grand Theft Auto: San Andreas leans clearly harder on optimization.
Diddy Kong Racing DS
Zipper routes and vehicle choice per track are learnable
Grand Theft Auto: San Andreas
Stats, gym, weapon skill, and routes reward systematic play
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.