head to head
Neither is meaningfully harder - they pose close to the same test. The widest gap is optimization, and it is a single point.

Execution · Competitive PvP
Stick skills and finishing wrapped around reading a human opponent's tendencies.

Execution · Competitive PvP
Arcade tennis with timing-based PvP mind games.
FIFA Soccer 09 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 09
Dribbling, timed tackles and finishing demand precise inputs
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 09
Human opponents shift tactics and exploit your habits mid-match
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 09
Rebounds and referee moments add minor randomness
Mario Tennis Aces
Mostly deterministic ball physics
FIFA Soccer 09 leans a little harder on optimization, though the gap is small.
FIFA Soccer 09
Squad selection and tactical setups are a solvable layer
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.