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 · Pure Mechanics
Free-to-play objective shooter from Splash Damage with mercenary classes.

Execution · Practiced Mastery
The sequel adding Horde mode to the campaign-plus-versus cover shooter formula.
Dirty Bomb leans Execution (Pure Mechanics); Gears of War 2 leans Execution (Practiced Mastery).
Dirty Bomb leans a little harder on execution, though the gap is small.
Dirty Bomb
Fast strafing gunfights and ability timing need quick hands
Gears of War 2
Gnasher battles and movement tech set the ceiling
Dirty Bomb leans a little harder on reads, though the gap is small.
Dirty Bomb
Human teams adapt pushes around objective chokepoints
Gears of War 2
Human opponents in versus drive prediction play
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Dirty Bomb
Fixed maps and abilities, minor loadout-card randomness
Gears of War 2
Horde spawns add mild randomness
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Dirty Bomb
Merc compositions and objective timings reward planning
Gears of War 2
Power-weapon timing and positioning are solvable
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.