head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; Me & My Katamari demands more optimization.

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

Optimization · Practiced Mastery
Tests routing an ever-growing ball through fixed stages against the clock.
Grand Theft Auto V leans Execution (Practiced Mastery); Me & My Katamari leans Optimization (Practiced Mastery).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Me & My Katamari
Awkward PSP rolling controls demand constant steering effort
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Me & My Katamari
Objects and people never adapt to the ball
Grand Theft Auto V leans a little harder on variance, though the gap is small.
Grand Theft Auto V
Some mission and lobby randomness.
Me & My Katamari
Object layouts are fixed per stage
Me & My Katamari leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
Me & My Katamari
Size-growth routing per stage has learnable optimal paths
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.