head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; Super Paper Mario demands more optimization.

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

Optimization · Practiced Mastery
A puzzle-leaning platformer testing perspective-flip problem solving more than reflexes.
Grand Theft Auto V leans Execution (Practiced Mastery); Super Paper Mario 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.
Super Paper Mario
Light platforming demands, well below mainline Mario
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Super Paper Mario
Enemies follow simple fixed patterns, no adaptation
Grand Theft Auto V leans a little harder on variance, though the gap is small.
Grand Theft Auto V
Some mission and lobby randomness.
Super Paper Mario
Dimension-flip secrets are scripted-but-unknown until found
Super Paper Mario leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
Super Paper Mario
Solving levels via the 2D/3D flip mechanic is the core test
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.