head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; Transformers: Devastation demands more execution.

Execution · Practiced Mastery
Sandbox crime playground spanning shooting, driving, and a persistent online economy.

Execution · Practiced Mastery
A Platinum-made cel-shaded brawler where the Autobots fight Megatron with stylish combo combat.
Grand Theft Auto V leans Execution (Practiced Mastery); Transformers: Devastation leans Execution (Practiced Mastery).
Transformers: Devastation leans clearly harder on execution.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Transformers: Devastation
Platinum dodge-offset timing drives the combat
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Transformers: Devastation
Decepticons run learnable scripted patterns
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Grand Theft Auto V
Some mission and lobby randomness.
Transformers: Devastation
Random weapon drops shape loadouts
Transformers: Devastation leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
Transformers: Devastation
Weapon synthesis, ability builds, boss moveset mastery
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.