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

Execution · Practiced Mastery
Tests arcade grapple timing with shallow systems underneath.

Execution · Competitive PvP
Reversal timing and pacing against an opponent baiting your counters.
WCW Nitro leans Execution (Practiced Mastery); WWE 2K15 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.
WCW Nitro
Fast grapple timing in an arcade-paced wrestling frame
WWE 2K15
Reversal timing windows decide most exchanges
WWE 2K15 leans a little harder on reads, though the gap is small.
WCW Nitro
Human versus play rewards mild move prediction
WWE 2K15
Human opponents bait reversals and vary move timing
WWE 2K15 leans a little harder on variance, though the gap is small.
WCW Nitro
Little randomness beyond AI behaviour
WWE 2K15
Chain-wrestling rock-paper-scissors minigames add randomness
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
WCW Nitro
Learning each wrestler's move list is quickly exhausted
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.