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
Stick execution and opponent prediction over a franchise-management layer.

Execution · Competitive PvP
Reversal timing and pacing against an opponent baiting your counters.
NHL 2K6 leans Execution (Practiced Mastery); WWE 2K15 leans Execution (Competitive PvP).
NHL 2K6 leans a little harder on execution, though the gap is small.
NHL 2K6
Shot timing, dekes and poke checks decide possessions
WWE 2K15
Reversal timing windows decide most exchanges
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
NHL 2K6
Human opponents disguise plays and bait pulls online
WWE 2K15
Human opponents bait reversals and vary move timing
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
NHL 2K6
Rebounds and deflections add per-game noise
WWE 2K15
Chain-wrestling rock-paper-scissors minigames add randomness
NHL 2K6 leans a little harder on optimization, though the gap is small.
NHL 2K6
Line management and franchise decisions reward planning
WWE 2K15
Move sets, stamina and match pacing are learnable systems
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.