head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; Titan Attacks! demands more execution.

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

Execution · Practiced Mastery
A Space Invaders descendant testing arcade aim plus an upgrade-economy layer.
Grand Theft Auto V leans Execution (Practiced Mastery); Titan Attacks! leans Execution (Practiced Mastery).
Titan Attacks! leans clearly harder on execution.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Titan Attacks!
Dodging fire while sniping descending waves is pure arcade aim
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Titan Attacks!
Alien waves follow fixed formations
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.
Titan Attacks!
Bounty drops and saucer timing add minor noise
Titan Attacks! leans a little harder on optimization, though the gap is small.
Grand Theft Auto V
Heists, money loops, builds learnable.
Titan Attacks!
Upgrade purchases between waves form a small economy to solve
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.