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

Optimization · Practiced Mastery
Solving fixed environmental puzzles by trial and error, with light timed stealth.

Execution · Practiced Mastery
Sandbox crime playground spanning shooting, driving, and a persistent online economy.
Black the Fall leans Optimization (Practiced Mastery); Grand Theft Auto V leans Execution (Practiced Mastery).
Grand Theft Auto V leans a little harder on execution, though the gap is small.
Black the Fall
Some timed stealth and chase sections demand light precision
Grand Theft Auto V
Driving and shooting demand decent inputs.
Grand Theft Auto V leans clearly harder on reads.
Black the Fall
Guards and machines run fixed detection scripts
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Black the Fall leans a little harder on variance, though the gap is small.
Black the Fall
Puzzle mechanics are unknown until each trial-and-error death
Grand Theft Auto V
Some mission and lobby randomness.
Black the Fall leans clearly harder on optimization.
Black the Fall
Each room is a fixed puzzle with one solution
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.