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 · Competitive PvP
Reversal timing and pacing against an opponent baiting your counters.

Execution · Competitive PvP
Tests grapple timing and predicting an opponent's next move choice.
WWE 2K15 leans Execution (Competitive PvP); WWF SmackDown! 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 2K15
Reversal timing windows decide most exchanges
WWF SmackDown!
Grapple timing and reversal inputs decide exchanges
WWF SmackDown! leans a little harder on reads, though the gap is small.
WWE 2K15
Human opponents bait reversals and vary move timing
WWF SmackDown!
Against a friend, predicting grapple choices is the game
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
WWE 2K15
Chain-wrestling rock-paper-scissors minigames add randomness
WWF SmackDown!
Some reversal and AI randomness in season mode
WWE 2K15 leans a little harder on optimization, though the gap is small.
WWE 2K15
Move sets, stamina and match pacing are learnable systems
WWF SmackDown!
Move-set knowledge and stamina management are shallow but real
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.