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

Execution · Competitive PvP
Futuristic large-scale Battlefield where teams besiege flying Titan carriers.

Execution · Pure Mechanics
Trilogy capstone whose equal-start arena multiplayer defined a console generation.
Battlefield 2142 leans Execution (Competitive PvP); Halo 3 leans Execution (Pure Mechanics).
Halo 3 leans a little harder on execution, though the gap is small.
Battlefield 2142
Gunplay, piloting, and Titan boarding need solid mechanics
Halo 3
Precision aim and grenade-melee timing
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Battlefield 2142
64-player human teams shift fronts and Titan assaults
Halo 3
Equal-start fights reward predicting humans
Battlefield 2142 leans a little harder on variance, though the gap is small.
Battlefield 2142
Large maps hide flanks; battle chaos obscures information
Halo 3
Equipment pickups and spawn variation
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Battlefield 2142
Unlock paths, squad roles, and Titan strategy reward planning
Halo 3
Power-weapon timers and map control
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.