head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; Suicide Squad: Kill the Justice League demands more optimization.

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

Optimization · Practiced Mastery
A looter-shooter loop testing build optimisation more than the shooting itself.
Grand Theft Auto V leans Execution (Practiced Mastery); Suicide Squad: Kill the Justice League leans Optimization (Practiced Mastery).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Suicide Squad: Kill the Justice League
Traversal-shooting needs moderate aim, generous by shooter standards
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Suicide Squad: Kill the Justice League
Brainwashed bosses are scripted encounters
Suicide Squad: Kill the Justice League leans a little harder on variance, though the gap is small.
Grand Theft Auto V
Some mission and lobby randomness.
Suicide Squad: Kill the Justice League
Loot rolls gate the live-service grind
Suicide Squad: Kill the Justice League leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
Suicide Squad: Kill the Justice League
Gear builds and mission farming dominate endgame
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.