head to head
Neither is simply harder - they are hard in different places. Borderlands: The Pre-Sequel demands more variance; Grand Theft Auto V demands more reads.

Optimization · Practiced Mastery
A moon-set Borderlands entry adding low gravity and oxygen mechanics.

Execution · Practiced Mastery
Sandbox crime playground spanning shooting, driving, and a persistent online economy.
Borderlands: The Pre-Sequel leans Optimization (Practiced Mastery); Grand Theft Auto V leans Execution (Practiced Mastery).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Borderlands: The Pre-Sequel
Low-gravity movement and aim are moderate demands
Grand Theft Auto V
Driving and shooting demand decent inputs.
Grand Theft Auto V leans clearly harder on reads.
Borderlands: The Pre-Sequel
Scripted enemies only
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Borderlands: The Pre-Sequel leans clearly harder on variance.
Borderlands: The Pre-Sequel
Loot RNG remains central
Grand Theft Auto V
Some mission and lobby randomness.
Borderlands: The Pre-Sequel leans clearly harder on optimization.
Borderlands: The Pre-Sequel
Oz-kit builds and gear synergy decide power
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.