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

Execution · Competitive PvP
Shot timing and play-calling against opponents who scheme around your habits.
Gears of War 2 leans Execution (Practiced Mastery); NBA 2K16 leans Execution (Competitive PvP).
Gears of War 2 leans a little harder on execution, though the gap is small.
Gears of War 2
Gnasher battles and movement tech set the ceiling
NBA 2K16
Shot-stick timing and dribble moves gate every possession
NBA 2K16 leans a little harder on reads, though the gap is small.
Gears of War 2
Human opponents in versus drive prediction play
NBA 2K16
Online opponents adjust defensive schemes against your tendencies
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Gears of War 2
Horde spawns add mild randomness
NBA 2K16
Shot RNG and card packs add bounded randomness
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Gears of War 2
Power-weapon timing and positioning are solvable
NBA 2K16
Playbooks, rotations and roster building reward deep study
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.