head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; M.U.S.H.A. demands more execution.

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

Execution · Practiced Mastery
Compile's mecha vertical shmup for the Genesis, famed for speed and configurable option pods.
Grand Theft Auto V leans Execution (Practiced Mastery); M.U.S.H.A. leans Execution (Practiced Mastery).
M.U.S.H.A. leans clearly harder on execution.
Grand Theft Auto V
Driving and shooting demand decent inputs.
M.U.S.H.A.
Fast vertical dodging through dense fire tests the hands.
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
M.U.S.H.A.
Enemies follow fixed choreography.
Grand Theft Auto V leans a little harder on variance, though the gap is small.
Grand Theft Auto V
Some mission and lobby randomness.
M.U.S.H.A.
Stages are fixed and memorisable.
M.U.S.H.A. leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
M.U.S.H.A.
Option formations and weapon choice reward system mastery.
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.