head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; Hexen II demands more optimization.

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

Execution · Practiced Mastery
Quake-powered dark-fantasy FPS continuing Hexen's puzzle-hub crusade across four worlds.
Grand Theft Auto V leans Execution (Practiced Mastery); Hexen II leans Execution (Practiced Mastery).
Hexen II leans a little harder on execution, though the gap is small.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Hexen II
Quake-engine combat and movement
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Hexen II
Scripted enemy AI
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.
Hexen II
Obscure puzzle triggers feel hidden first run
Hexen II leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
Hexen II
Hub puzzles, class builds, and route knowledge
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.