head to head
Neither is meaningfully harder - they pose close to the same test. The widest gap is reads, and it is a single point.

Execution · Practiced Mastery
Landmark scripted FPS following Gordon Freeman through the Black Mesa disaster.

Execution · Practiced Mastery
Moody Halo side story about ordinary troopers piecing together a night in New Mombasa.
Half-Life leans Execution (Practiced Mastery); Halo 3: ODST 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.
Half-Life
Combat aim and infamous platforming sections
Halo 3: ODST
Squishier soldier demands careful aim and positioning
Halo 3: ODST leans a little harder on reads, though the gap is small.
Half-Life
Marines flank by script, not by modelling you
Halo 3: ODST
Covenant patrols are scripted, not adaptive
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Half-Life
Set pieces surprise only on first run
Halo 3: ODST
Open Mombasa hides clues until found
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Half-Life
Resource routing and encounter knowledge for speedruns
Halo 3: ODST
Firefight wave knowledge and route planning
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.