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
Stick execution and outguessing a human defender, with tactics underneath.

Execution · Competitive PvP
Arcade tennis with timing-based PvP mind games.
FIFA Soccer 08 leans Execution (Competitive PvP); Mario Tennis Aces 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.
FIFA Soccer 08
Dribbling, timing tackles and finishing reward stick skill
Mario Tennis Aces
Shot timing, charge mechanics, footwork
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
FIFA Soccer 08
Versus humans, baiting defenders and disguising attacks is core
Mario Tennis Aces
Online tennis demands reading opponent placement
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
FIFA Soccer 08
Ball physics adds bounce noise to single matches
Mario Tennis Aces
Mostly deterministic ball physics
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
FIFA Soccer 08
Formations and squad selection matter but stay shallow
Mario Tennis Aces
Character matchups and tech routes
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.