head to head
Neither is harder. They score identically on all four axes, so the difference between them is in kind, not degree.

Execution · Practiced Mastery
Moody Halo side story about ordinary troopers piecing together a night in New Mombasa.

Execution · Practiced Mastery
The sequel-remake pairing Butcher Bay with a new stealth-action campaign aboard a mercenary ship.
Halo 3: ODST leans Execution (Practiced Mastery); The Chronicles of Riddick: Assault on Dark Athena leans Execution (Practiced Mastery).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Halo 3: ODST
Squishier soldier demands careful aim and positioning
The Chronicles of Riddick: Assault on Dark Athena
Melee takedowns and gunplay demand reliable hands
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Halo 3: ODST
Covenant patrols are scripted, not adaptive
The Chronicles of Riddick: Assault on Dark Athena
Drone and guard behavior is fully scripted
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Halo 3: ODST
Open Mombasa hides clues until found
The Chronicles of Riddick: Assault on Dark Athena
First-run uncertainty about patrols and drone placements
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Halo 3: ODST
Firefight wave knowledge and route planning
The Chronicles of Riddick: Assault on Dark Athena
Stealth routes and encounter solutions are learnable fixed content
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.