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
Tests quick ball control and outguessing a human defender's tackles.

Execution · Competitive PvP
Arcade tennis with timing-based PvP mind games.
International Superstar Soccer Deluxe leans Execution (Competitive PvP); Mario Tennis Aces leans Execution (Competitive PvP).
Mario Tennis Aces leans a little harder on execution, though the gap is small.
International Superstar Soccer Deluxe
Pass and shot timing under pressure decide matches
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.
International Superstar Soccer Deluxe
Versus a human, faking runs and reading tackles is central
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.
International Superstar Soccer Deluxe
Deflections and keeper bounces add light chaos
Mario Tennis Aces
Mostly deterministic ball physics
International Superstar Soccer Deluxe leans a little harder on optimization, though the gap is small.
International Superstar Soccer Deluxe
Formations and set-piece routines are learnable systems
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.