head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; Terra Nova: Strike Force Centauri demands more optimization.

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

Execution · Practiced Mastery
Looking Glass's 1996 squad-command sim of powered-armor combat on alien battlefields.
Grand Theft Auto V leans Execution (Practiced Mastery); Terra Nova: Strike Force Centauri leans Execution (Practiced Mastery).
Terra Nova: Strike Force Centauri leans a little harder on execution, though the gap is small.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Terra Nova: Strike Force Centauri
Piloting powered armor and aiming under fire is demanding
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Terra Nova: Strike Force Centauri
Enemy forces follow mission scripts, never adapt
Terra Nova: Strike Force Centauri leans a little harder on variance, though the gap is small.
Grand Theft Auto V
Some mission and lobby randomness.
Terra Nova: Strike Force Centauri
Unknown enemy placements on first mission attempts
Terra Nova: Strike Force Centauri leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
Terra Nova: Strike Force Centauri
Squad orders, loadouts and approach routes are plannable
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.