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
Arcade wrestling that tests reversal timing and little else by design.

Execution · Competitive PvP
Reversal timing and pacing against an opponent baiting your counters.
WWE 2K Battlegrounds 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 2K Battlegrounds
Button-mash combos and reversal timing set the pace
WWE 2K15
Reversal timing windows decide most exchanges
WWE 2K15 leans a little harder on reads, though the gap is small.
WWE 2K Battlegrounds
Human opponents add light reversal mind games
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 2K Battlegrounds
Power-ups and interactive arenas inject party chaos
WWE 2K15
Chain-wrestling rock-paper-scissors minigames add randomness
WWE 2K15 leans a little harder on optimization, though the gap is small.
WWE 2K Battlegrounds
Shallow systems - move knowledge helps a little
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.