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 · Competitive PvP
Football execution and human mind-games layered over tactical formation play.

Execution · Competitive PvP
Reversal timing and pacing against an opponent baiting your counters.
International Superstar Soccer Pro 98 leans Execution (Competitive PvP); WWE 2K15 leans Execution (Competitive PvP).
International Superstar Soccer Pro 98 leans a little harder on execution, though the gap is small.
International Superstar Soccer Pro 98
First-touch timing and finishing demand practiced hands
WWE 2K15
Reversal timing windows decide most exchanges
International Superstar Soccer Pro 98 leans a little harder on reads, though the gap is small.
International Superstar Soccer Pro 98
Versus matches hinge on outguessing a human defender
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.
International Superstar Soccer Pro 98
Bounces and deflections add bounded match chaos
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.
International Superstar Soccer Pro 98
Formations and set-piece routines have learnable best answers
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.