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
Cover-based shotgun duels testing movement execution and positional prediction in multiplayer.

Execution · Pure Mechanics
Trilogy capstone whose equal-start arena multiplayer defined a console generation.
Gears of War: Ultimate Edition leans Execution (Competitive PvP); Halo 3 leans Execution (Pure Mechanics).
Halo 3 leans a little harder on execution, though the gap is small.
Gears of War: Ultimate Edition
Wallbounce movement and gnasher duels demand real hands
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.
Gears of War: Ultimate Edition
Multiplayer is constant positional mind-games around cover
Halo 3
Equal-start fights reward predicting humans
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Gears of War: Ultimate Edition
Weapon spawns timed and known; spread mildly random
Halo 3
Equipment pickups and spawn variation
Halo 3 leans a little harder on optimization, though the gap is small.
Gears of War: Ultimate Edition
Map control and spawn timing knowledge structure matches
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.