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 · Competitive PvP
Tests shot timing and play-calling prediction against human defenders.

Execution · Competitive PvP
Shot timing and play-calling against opponents who scheme around your habits.
NBA 2K15 leans Execution (Competitive PvP); NBA 2K16 leans Execution (Competitive PvP).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
NBA 2K15
Shot-stick timing and dribble moves demand practiced hands
NBA 2K16
Shot-stick timing and dribble moves gate every possession
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
NBA 2K15
Versus humans, baiting defenders and calling counters drives possessions
NBA 2K16
Online opponents adjust defensive schemes against your tendencies
NBA 2K15 leans a little harder on variance, though the gap is small.
NBA 2K15
Shot RNG and loose balls swing close games
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.
NBA 2K15
Playbooks, matchups and roster management reward system study
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.