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

Execution · Practiced Mastery
Zelda-meets-God-of-War action adventure starring the horseman War.

Execution · Practiced Mastery
Sandbox crime playground spanning shooting, driving, and a persistent online economy.
Darksiders leans Execution (Practiced Mastery); Grand Theft Auto V leans Execution (Practiced Mastery).
Darksiders leans a little harder on execution, though the gap is small.
Darksiders
combo-driven combat with dodge timing
Grand Theft Auto V
Driving and shooting demand decent inputs.
Grand Theft Auto V leans clearly harder on reads.
Darksiders
enemies are scripted
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.
Darksiders
world and bosses are fixed
Grand Theft Auto V
Some mission and lobby randomness.
Darksiders leans clearly harder on optimization.
Darksiders
Zelda-style dungeons and upgrade routing
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.