head to head
Neither is simply harder - they are hard in different places. 12 is Better Than 6 demands more execution; Grand Theft Auto V demands more reads.

Execution · Practiced Mastery
A hand-drawn western take on Hotline Miami where you cock the revolver before every single shot.

Execution · Practiced Mastery
Sandbox crime playground spanning shooting, driving, and a persistent online economy.
12 is Better Than 6 leans Execution (Practiced Mastery); Grand Theft Auto V leans Execution (Practiced Mastery).
12 is Better Than 6 leans clearly harder on execution.
12 is Better Than 6
One-hit deaths, manual cocking, fast top-down aim
Grand Theft Auto V
Driving and shooting demand decent inputs.
Grand Theft Auto V leans clearly harder on reads.
12 is Better Than 6
Enemies are placed and scripted
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
12 is Better Than 6
Unknown enemy placements on first attempts only
Grand Theft Auto V
Some mission and lobby randomness.
12 is Better Than 6 leans a little harder on optimization, though the gap is small.
12 is Better Than 6
Each room becomes a learnable clear order
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.