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

Execution · Competitive PvP
Mechanical football control plus predicting how a human opponent attacks.

Execution · Competitive PvP
Arcade tennis with timing-based PvP mind games.
FIFA 16 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 16
Dribbling, defending and finishing inputs decide duels
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 16
Human opponents online adapt tactics match to match
Mario Tennis Aces
Online tennis demands reading opponent placement
FIFA 16 leans a little harder on variance, though the gap is small.
FIFA 16
Deflections and finishing randomness swing close games
Mario Tennis Aces
Mostly deterministic ball physics
FIFA 16 leans a little harder on optimization, though the gap is small.
FIFA 16
Formations and squad construction reward study
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.