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

Execution · Practiced Mastery
The sequel adding Horde mode to the campaign-plus-versus cover shooter formula.

Execution · Pure Mechanics
Console shooter execution across scripted set pieces and once-massive human multiplayer.
Gears of War 2 leans Execution (Practiced Mastery); Resistance 2 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.
Gears of War 2
Gnasher battles and movement tech set the ceiling
Resistance 2
Weapon handling and boss dodging demand console-shooter precision
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Gears of War 2
Human opponents in versus drive prediction play
Resistance 2
60-player multiplayer rewarded reading humans; campaign Chimera don't adapt
Gears of War 2 leans a little harder on variance, though the gap is small.
Gears of War 2
Horde spawns add mild randomness
Resistance 2
Campaign encounters are set pieces
Gears of War 2 leans a little harder on optimization, though the gap is small.
Gears of War 2
Power-weapon timing and positioning are solvable
Resistance 2
Exotic weapon choice and class co-op roles add 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.