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 · Competitive PvP
Vietnam-era large-scale multiplayer shooter mixing infantry firefights with helicopters and jungle ambushes.

Execution · Pure Mechanics
Heavy first-person gunplay testing aim, with positional prediction in multiplayer.
Battlefield Vietnam leans Execution (Competitive PvP); Killzone 3 leans Execution (Pure Mechanics).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Battlefield Vietnam
Gunplay, vehicle handling, and helicopter piloting
Killzone 3
Weighty gunplay still demands aim and recoil control
Battlefield Vietnam leans a little harder on reads, though the gap is small.
Battlefield Vietnam
Human squads flank and contest objectives unpredictably
Killzone 3
Multiplayer opponents flank and bait; campaign AI is scripted
Battlefield Vietnam leans a little harder on variance, though the gap is small.
Battlefield Vietnam
Spawn waves and vehicle availability shuffle fights
Killzone 3
Spawns and spread inject limited randomness
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Battlefield Vietnam
Map control, loadouts, and ticket-bleed knowledge
Killzone 3
Map control, loadouts and class roles structure multiplayer
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.