head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; Trine 3: The Artifacts of Power demands more optimization.

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

Optimization · Practiced Mastery
Solving physics puzzles by picking the right hero ability for each obstacle.
Grand Theft Auto V leans Execution (Practiced Mastery); Trine 3: The Artifacts of Power 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.
Trine 3: The Artifacts of Power
Light platforming and physics manipulation, forgiving timing windows
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Trine 3: The Artifacts of Power
No adaptive opponents - puzzles and scripted enemies only
Grand Theft Auto V leans a little harder on variance, though the gap is small.
Grand Theft Auto V
Some mission and lobby randomness.
Trine 3: The Artifacts of Power
Levels are fixed and fully visible
Trine 3: The Artifacts of Power leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
Trine 3: The Artifacts of Power
Choosing the right hero and physics solution is the game
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.