head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; Mad Dog McCree demands more execution.

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

Execution · Practiced Mastery
Lightgun quick-draw reflexes against memorisable filmed ambush sequences.
Grand Theft Auto V leans Execution (Practiced Mastery); Mad Dog McCree leans Execution (Practiced Mastery).
Mad Dog McCree leans clearly harder on execution.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Mad Dog McCree
Quick-draw lightgun accuracy under strict timers
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Mad Dog McCree
Outlaws appear in filmed, fixed sequences
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Grand Theft Auto V
Some mission and lobby randomness.
Mad Dog McCree
Some ambush order shuffling, quickly mapped
Mad Dog McCree leans a little harder on optimization, though the gap is small.
Grand Theft Auto V
Heists, money loops, builds learnable.
Mad Dog McCree
Memorising who pops out where is the real progression
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.