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

Execution · Pure Mechanics
WWII gunfight execution with light multiplayer prediction.

Execution · Competitive PvP
Fast arcade football execution with couch-multiplayer mind games.
Call of Duty 3 leans Execution (Pure Mechanics); FIFA: Road to World Cup 98 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.
Call of Duty 3
Aimbot trivialises both campaign and multiplayer gunfights
FIFA: Road to World Cup 98
Arcade-paced dribbling and shot timing decide matches
FIFA: Road to World Cup 98 leans a little harder on reads, though the gap is small.
Call of Duty 3
Multiplayer adds modest human positional prediction
FIFA: Road to World Cup 98
Couch opponents can be baited; AI cannot
FIFA: Road to World Cup 98 leans a little harder on variance, though the gap is small.
Call of Duty 3
Scripted setpieces and spawn flow add minor unknowns
FIFA: Road to World Cup 98
Bouncy ball physics inject classic FIFA luck
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Call of Duty 3
Map knowledge and weapon choice reward light study
FIFA: Road to World Cup 98
Simple era tactics and exploit goals are quickly mapped
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.