head to head
Neither is simply harder - they are hard in different places. Crisis Core: Final Fantasy VII demands more variance; Grand Theft Auto V demands more reads.

Optimization · Practiced Mastery
Action RPG prequel to FFVII whose combat spins a slot machine for buffs.

Execution · Practiced Mastery
Sandbox crime playground spanning shooting, driving, and a persistent online economy.
Crisis Core: Final Fantasy VII 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.
Crisis Core: Final Fantasy VII
real-time dodging and attack timing
Grand Theft Auto V
Driving and shooting demand decent inputs.
Grand Theft Auto V leans clearly harder on reads.
Crisis Core: Final Fantasy VII
no adaptive opponent
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Crisis Core: Final Fantasy VII leans clearly harder on variance.
Crisis Core: Final Fantasy VII
DMW slot reels randomize limit breaks and levels
Grand Theft Auto V
Some mission and lobby randomness.
Crisis Core: Final Fantasy VII leans clearly harder on optimization.
Crisis Core: Final Fantasy VII
materia fusion is a deep solvable system
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.