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

Optimization · Practiced Mastery
The original loot-driven co-op shooter set on the bandit world of Pandora.

Execution · Practiced Mastery
Sandbox crime playground spanning shooting, driving, and a persistent online economy.
Borderlands 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
Standard FPS aim; numbers matter more than precision
Grand Theft Auto V
Driving and shooting demand decent inputs.
Grand Theft Auto V leans clearly harder on reads.
Borderlands
Bandits and skags are scripted
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Borderlands leans clearly harder on variance.
Borderlands
Procedural loot drops gate your power
Grand Theft Auto V
Some mission and lobby randomness.
Borderlands leans clearly harder on optimization.
Borderlands
Builds, gear math, and farming routes dominate
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.