head to head
Neither is simply harder - they are hard in different places. Call of Juarez: Gunslinger demands more execution; Grand Theft Auto V demands more reads.

Execution · Practiced Mastery
Arcade western shooter narrated as a bounty hunter's tall tale.

Execution · Practiced Mastery
Sandbox crime playground spanning shooting, driving, and a persistent online economy.
Call of Juarez: Gunslinger leans Execution (Practiced Mastery); Grand Theft Auto V leans Execution (Practiced Mastery).
Call of Juarez: Gunslinger leans clearly harder on execution.
Call of Juarez: Gunslinger
Quick-draw duels and combo shooting demand precision
Grand Theft Auto V
Driving and shooting demand decent inputs.
Grand Theft Auto V leans clearly harder on reads.
Call of Juarez: Gunslinger
Enemies are scripted gallery targets
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Grand Theft Auto V leans a little harder on variance, though the gap is small.
Call of Juarez: Gunslinger
Minor enemy placement surprises first run
Grand Theft Auto V
Some mission and lobby randomness.
Call of Juarez: Gunslinger leans clearly harder on optimization.
Call of Juarez: Gunslinger
Score routes, skill trees, duel timing knowledge
Grand Theft Auto V
Heists, money loops, builds learnable.
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.