head to head
Neither is harder. They score identically on all four axes, so the difference between them is in kind, not degree.

Execution · Competitive PvP
Reversal-timing duels dressed in wrestling spectacle, with light opponent prediction.

Execution · Competitive PvP
Reversal timing and pacing against an opponent baiting your counters.
WWE 2K14 leans Execution (Competitive PvP); 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.
WWE 2K14
Reversal timing windows are the main physical test
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.
WWE 2K14
Human opponents bait reversals; AI is mappable
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.
WWE 2K14
Comeback mechanics add scripted swing
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.
WWE 2K14
Knowing move properties and match-type rules pays off
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.